diff --git a/API.md b/API.md
index e564830..ef9ae9a 100644
--- a/API.md
+++ b/API.md
@@ -618,6 +618,49 @@ Remains `undefined` if there is no attribute value.
 
 Returns the attribute name qualified with the namespace if one is given.
 
+### `attribute.insensitive` and `attribute.sensitive`
+
+The two case sensitivity flags defined by
+[Selectors Level 4](https://www.w3.org/TR/selectors-4/#attribute-case):
+
+* `attribute.insensitive` is `true` for the case insensitive flag, `[href="foo" i]`.
+* `attribute.sensitive` is `true` for the explicit case sensitive flag, `[href="foo" s]`.
+
+Both remain `undefined` if the attribute carries no flag at all.
+
+`attribute.insensitiveFlag` and `attribute.sensitiveFlag` are read-only and return
+the canonical lowercase flag (`"i"` / `"s"`) when the corresponding boolean is set,
+otherwise an empty string.
+
+```css
+[href="foo"]   /* insensitive: undefined, sensitive: undefined */
+[href="foo" i] /* insensitive: true,      sensitive: false     */
+[href="foo" s] /* insensitive: false,     sensitive: true      */
+```
+
+A selector cannot request both case sensitivities at once, so the two properties
+are mutually exclusive: setting either one to `true` clears the other, along with
+any capitalized notation stored for it in
+[`attribute.raws`](#attributeraws).
+
+```js
+let attr = parser().astSync('[href="foo" I]').first.first;
+attr.sensitive = true;
+attr.insensitive; // false
+attr.raws.insensitiveFlag; // undefined
+attr.toString(); // '[href="foo" s]'
+```
+
+Because a flag can only ever appear in one place in the output, the two flags
+share a single slot. The whitespace and comments around either flag live in
+`attribute.spaces.insensitive` and `attribute.raws.spaces.insensitive`, and
+`attribute.offsetOf()` reports the position of that slot under whichever of
+`"insensitive"` or `"sensitive"` is actually present.
+
+Non-standard flags are not modeled as booleans. They are preserved verbatim in
+`attribute.raws.insensitiveFlag` (see [`attribute.raws`](#attributeraws)) and are
+left untouched by both setters.
+
 ### `attribute.offsetOf(part)`
 
  Returns the offset of the attribute part specified relative to the
@@ -637,6 +680,10 @@ Returns the attribute name qualified with the namespace if one is given.
  * `"operator"` - the match operator of the attribute
  * `"value"` - The value (string or identifier)
  * `"insensitive"` - the case insensitivity flag
+ * `"sensitive"` - the explicit case sensitivity flag
+
+ The two case sensitivity flags share one slot in the output, so at most one of
+ `"insensitive"` and `"sensitive"` resolves to an offset; the other returns `-1`.
 
 ### `attribute.raws.unquoted`
 
@@ -675,6 +722,18 @@ or `value` then a property is placed in the raws for that value containing the f
 If a comment is embedded within the space between parts of the attribute
 then the raw for that space is set accordingly.
 
+The case sensitivity flags are stored here when the source used a notation that
+does not round-trip from the boolean alone:
+
+* `attribute.raws.insensitiveFlag` holds `"I"` for `[href="foo" I]`. It also holds
+  any non-standard flag verbatim — `"y"` for `[href="foo" y]` — which is how such
+  a flag survives serialization even though neither `attribute.insensitive` nor
+  `attribute.sensitive` is `true`.
+* `attribute.raws.sensitiveFlag` holds `"S"` for `[href="foo" S]`.
+
+The canonical lowercase forms are not stored, since `attribute.insensitive` and
+`attribute.sensitive` already render them.
+
 Setting an attribute's property `raws` value to be deleted.
 
 For now, changing the spaces required also updating or removing any of the
diff --git a/postcss-selector-parser.d.ts b/postcss-selector-parser.d.ts
index 9f0a233..ecae141 100644
--- a/postcss-selector-parser.d.ts
+++ b/postcss-selector-parser.d.ts
@@ -387,6 +387,7 @@ declare namespace parser {
         attribute: string;
         operator?: AttributeOperator;
         insensitive?: boolean;
+        sensitive?: boolean;
         quoteMark?: QuoteMark;
         /** @deprecated Use quoteMark instead. */
         quoted?: boolean;
@@ -404,6 +405,8 @@ declare namespace parser {
             operator?: string;
             value?: string;
             insensitive?: string;
+            insensitiveFlag?: string;
+            sensitiveFlag?: string;
             spaces?: {
                 attribute?: Partial<Spaces>;
                 operator?: Partial<Spaces>;
@@ -417,6 +420,7 @@ declare namespace parser {
         attribute: string;
         operator?: AttributeOperator;
         insensitive?: boolean;
+        sensitive?: boolean;
         quoteMark: QuoteMark;
         quoted?: boolean;
         spaces: {
@@ -435,6 +439,8 @@ declare namespace parser {
             /** The value of the attribute with quotes and escapes. */
             value?: string;
             insensitive?: string;
+            insensitiveFlag?: string;
+            sensitiveFlag?: string;
             spaces?: {
                 attribute?: Partial<Spaces>;
                 operator?: Partial<Spaces>;
@@ -453,6 +459,12 @@ declare namespace parser {
          */
         readonly insensitiveFlag : 'i' | '';
 
+        /**
+         * The explicit case sensitivity flag (Selectors Level 4 `s`) or an empty
+         * string depending on whether this attribute is explicitly case sensitive.
+         */
+        readonly sensitiveFlag : 's' | '';
+
         /**
          * Returns the attribute's value quoted such that it would be legal to use
          * in the value of a css file. The original value's quotation setting
@@ -505,7 +517,7 @@ declare namespace parser {
          * @param part One of the possible values inside an attribute.
          * @returns -1 if the name is invalid or the value doesn't exist in this attribute.
          */
-        offsetOf(part: "ns" | "namespace" | "attribute" | "attributeNS" | "operator" | "value" | "insensitive"): number;
+        offsetOf(part: "ns" | "namespace" | "attribute" | "attributeNS" | "operator" | "value" | "insensitive" | "sensitive"): number;
     }
     function attribute(opts: AttributeOptions): Attribute;
     function isAttribute(node: any): node is Attribute;
diff --git a/src/__tests__/attributes.mjs b/src/__tests__/attributes.mjs
index 47caf20..d18dcd5 100644
--- a/src/__tests__/attributes.mjs
+++ b/src/__tests__/attributes.mjs
@@ -487,6 +487,122 @@ test("non standard modifiers", '[href="foo" y]', (t, tree) => {
   t.deepEqual(tree.toString(), '[href="foo" y]');
 });
 
+// https://github.com/postcss/postcss-selector-parser/issues/309
+test("case sensitive attribute selector", '[href="foo" s]', (t, tree) => {
+  let attr = tree.atPosition(1, 13);
+  t.deepEqual(attr.sensitive, true);
+  t.deepEqual(attr.sensitiveFlag, "s");
+  t.deepEqual(attr.insensitive, false);
+  t.deepEqual(attr.insensitiveFlag, "");
+  // The `s` flag must be modeled explicitly, not mis-filed into raws.insensitiveFlag.
+  t.is(attr.raws.insensitiveFlag, undefined);
+  // Both flags occupy the same slot, so only the one that is present resolves.
+  t.deepEqual(attr.offsetOf("sensitive"), 12);
+  t.deepEqual(attr.offsetOf("insensitive"), -1);
+  t.deepEqual(tree.toString(), '[href="foo" s]');
+});
+
+test("case insensitive attribute selector offset", '[href="foo" i]', (t, tree) => {
+  let attr = tree.atPosition(1, 13);
+  t.deepEqual(attr.offsetOf("insensitive"), 12);
+  t.deepEqual(attr.offsetOf("sensitive"), -1);
+});
+
+test("case sensitivity flags are mutually exclusive (i to s)", '[href="foo" i]', (t, tree) => {
+  let attr = tree.atPosition(1, 13);
+  t.deepEqual(attr.insensitive, true);
+
+  attr.sensitive = true;
+
+  // `[href="foo" i s]` is not valid CSS, so adopting one flag drops the other.
+  t.deepEqual(attr.sensitive, true);
+  t.deepEqual(attr.insensitive, false);
+  t.deepEqual(attr.sensitiveFlag, "s");
+  t.deepEqual(attr.insensitiveFlag, "");
+  t.deepEqual(tree.toString(), '[href="foo" s]');
+});
+
+test("case sensitivity flags are mutually exclusive (s to i)", '[href="foo" s]', (t, tree) => {
+  let attr = tree.atPosition(1, 13);
+  t.deepEqual(attr.sensitive, true);
+
+  attr.insensitive = true;
+
+  t.deepEqual(attr.insensitive, true);
+  t.deepEqual(attr.sensitive, false);
+  t.deepEqual(attr.insensitiveFlag, "i");
+  t.deepEqual(attr.sensitiveFlag, "");
+  t.deepEqual(tree.toString(), '[href="foo" i]');
+});
+
+test(
+  "switching case sensitivity erases the original notation (I to s)",
+  '[href="foo" I]',
+  (t, tree) => {
+    let attr = tree.atPosition(1, 13);
+    t.deepEqual(attr.raws.insensitiveFlag, "I");
+
+    attr.sensitive = true;
+
+    t.is(attr.raws.insensitiveFlag, undefined);
+    t.deepEqual(tree.toString(), '[href="foo" s]');
+  },
+);
+
+test(
+  "switching case sensitivity erases the original notation (S to i)",
+  '[href="foo" S]',
+  (t, tree) => {
+    let attr = tree.atPosition(1, 13);
+    t.deepEqual(attr.raws.sensitiveFlag, "S");
+
+    attr.insensitive = true;
+
+    t.is(attr.raws.sensitiveFlag, undefined);
+    t.deepEqual(tree.toString(), '[href="foo" i]');
+  },
+);
+
+test("case sensitivity setters keep non standard modifiers", '[href="foo" y]', (t, tree) => {
+  let attr = tree.atPosition(1, 13);
+  t.deepEqual(attr.raws.insensitiveFlag, "y");
+
+  // Only the standard "i"/"I" notations are erased, so an unrecognised flag kept
+  // in raws survives the round trip rather than being silently destroyed.
+  attr.sensitive = true;
+  t.deepEqual(attr.raws.insensitiveFlag, "y");
+  t.deepEqual(tree.toString(), '[href="foo" s]');
+
+  attr.sensitive = false;
+  t.deepEqual(attr.raws.insensitiveFlag, "y");
+  t.deepEqual(tree.toString(), '[href="foo" y]');
+});
+
+test("capitalized case sensitive attribute selector", '[href="foo" S]', (t, tree) => {
+  let attr = tree.atPosition(1, 13);
+  t.deepEqual(attr.sensitive, true);
+  t.deepEqual(attr.sensitiveFlag, "s");
+  t.deepEqual(attr.insensitive, false);
+  t.is(attr.raws.insensitiveFlag, undefined);
+  t.deepEqual(attr.raws.sensitiveFlag, "S");
+  t.deepEqual(tree.toString(), '[href="foo" S]');
+
+  // Clearing the flag must erase the original "S" notation from raws.
+  attr.sensitive = false;
+  t.is(attr.raws.sensitiveFlag, undefined);
+  t.deepEqual(tree.toString(), '[href="foo" ]');
+});
+
+test("case sensitive attribute selector (unquoted)", "[href=test s]", (t, tree) => {
+  let attr = tree.nodes[0].nodes[0];
+  t.deepEqual(attr.value, "test");
+  t.deepEqual(attr.sensitive, true);
+
+  attr.sensitive = false;
+
+  t.deepEqual(tree.toString(), "[href=test ]");
+});
+
 test("comment after insensitive(non space)", '[href="foo" i/**/]', (t, tree) => {
   // https://github.com/postcss/postcss-selector-parser/issues/150
   let attr = tree.atPosition(1, 13);
diff --git a/src/parser.js b/src/parser.js
index 3b00836..60ab84e 100644
--- a/src/parser.js
+++ b/src/parser.js
@@ -319,12 +319,21 @@ export default class Parser {
             lastAdded = "value";
           } else {
             let insensitive = content === "i" || content === "I";
+            let sensitive = content === "s" || content === "S";
             if (
               (node.value || node.value === "") &&
               (node.quoteMark || spaceAfterMeaningfulToken)
             ) {
               node.insensitive = insensitive;
-              if (!insensitive || content === "I") {
+              node.sensitive = sensitive;
+              if (sensitive) {
+                // "s" is the canonical case-sensitive flag; store the original
+                // notation in "raws.sensitiveFlag" only for the "S" variant.
+                if (content === "S") {
+                  ensureObject(node, "raws");
+                  node.raws.sensitiveFlag = content;
+                }
+              } else if (!insensitive || content === "I") {
                 ensureObject(node, "raws");
                 node.raws.insensitiveFlag = content;
               }
diff --git a/src/selectors/attribute.js b/src/selectors/attribute.js
index f97d994..1ab5e2f 100644
--- a/src/selectors/attribute.js
+++ b/src/selectors/attribute.js
@@ -244,6 +244,10 @@ export default class Attribute extends Namespace {
     return this.insensitive ? "i" : "";
   }
 
+  get sensitiveFlag() {
+    return this.sensitive ? "s" : "";
+  }
+
   get value() {
     return this._value;
   }
@@ -257,22 +261,58 @@ export default class Attribute extends Namespace {
    * If the case insensitive flag changes, the raw (escaped) value at `attr.raws.insensitiveFlag`
    * of the attribute is updated accordingly.
    *
+   * The two case sensitivity flags are mutually exclusive, so setting this flag
+   * clears `attr.sensitive` and any `s`/`S` in `attr.raws.sensitiveFlag`.
+   *
    * @param {true | false} insensitive true if the attribute should match case-insensitively.
    */
   set insensitive(insensitive) {
-    if (!insensitive) {
-      this._insensitive = false;
-
+    if (insensitive) {
+      // `[attr=value i s]` is not a thing, so adopting the case-insensitive flag
+      // drops the explicit case-sensitive flag and its original notation.
+      this.sensitive = false;
+    } else if (
+      this.raws &&
+      (this.raws.insensitiveFlag === "I" || this.raws.insensitiveFlag === "i")
+    ) {
       // "i" and "I" can be used in "this.raws.insensitiveFlag" to store the original notation.
       // When setting `attr.insensitive = false` both should be erased to ensure correct serialization.
-      if (this.raws && (this.raws.insensitiveFlag === "I" || this.raws.insensitiveFlag === "i")) {
-        this.raws.insensitiveFlag = undefined;
-      }
+      this.raws.insensitiveFlag = undefined;
     }
 
     this._insensitive = insensitive;
   }
 
+  get sensitive() {
+    return this._sensitive;
+  }
+
+  /**
+   * Set the explicit case sensitive flag (Selectors Level 4 `s`).
+   * If the case sensitive flag changes, the raw (escaped) value at `attr.raws.sensitiveFlag`
+   * of the attribute is updated accordingly.
+   *
+   * The two case sensitivity flags are mutually exclusive, so setting this flag
+   * clears `attr.insensitive` and any `i`/`I` in `attr.raws.insensitiveFlag`.
+   *
+   * @param {true | false} sensitive true if the attribute should match case-sensitively.
+   */
+  set sensitive(sensitive) {
+    if (sensitive) {
+      // `[attr=value s i]` is not a thing, so adopting the case-sensitive flag
+      // drops the case-insensitive flag and its original notation. Only the
+      // standard "i"/"I" notations are erased, so a non-standard flag kept in
+      // "this.raws.insensitiveFlag" survives and can still be round-tripped.
+      this.insensitive = false;
+    } else if (this.raws && (this.raws.sensitiveFlag === "S" || this.raws.sensitiveFlag === "s")) {
+      // "s" and "S" can be used in "this.raws.sensitiveFlag" to store the original notation.
+      // When setting `attr.sensitive = false` both should be erased to ensure correct serialization.
+      this.raws.sensitiveFlag = undefined;
+    }
+
+    this._sensitive = sensitive;
+  }
+
   /**
    * Before 3.0, the value had to be set to an escaped value including any wrapped
    * quote marks. In 3.0, the semantics of `Attribute.value` changed so that the value
@@ -345,6 +385,7 @@ export default class Attribute extends Namespace {
    * * "operator" - the match operator of the attribute
    * * "value" - The value (string or identifier)
    * * "insensitive" - the case insensitivity flag;
+   * * "sensitive" - the explicit case sensitivity flag;
    * @param part One of the possible values inside an attribute.
    * @returns -1 if the name is invalid or the value doesn't exist in this attribute.
    */
@@ -387,11 +428,17 @@ export default class Attribute extends Namespace {
 
     count += value.length;
     count += valueSpaces.after.length;
+    // Both case sensitivity flags occupy the same slot after the value, so they
+    // share the "insensitive" spaces. Each name only resolves when its own flag
+    // is the one actually present.
     let insensitiveSpaces = this._spacesFor("insensitive");
     count += insensitiveSpaces.before.length;
     if (name === "insensitive") {
       return this.insensitive ? count : -1;
     }
+    if (name === "sensitive") {
+      return this.sensitive ? count : -1;
+    }
     return -1;
   }
 
@@ -403,8 +450,13 @@ export default class Attribute extends Namespace {
     if (this.operator && (this.value || this.value === "")) {
       selector.push(this._stringFor("operator"));
       selector.push(this._stringFor("value"));
+      // The case-sensitivity flag occupies a single slot after the value.
+      // Prefer the explicit case-sensitive flag ("s"/"S") when present,
+      // otherwise fall back to the case-insensitive flag ("i"/"I") or any
+      // non-standard flag preserved in `raws.insensitiveFlag`.
+      let flagProperty = this.sensitive ? "sensitiveFlag" : "insensitiveFlag";
       selector.push(
-        this._stringFor("insensitiveFlag", "insensitive", (attrValue, attrSpaces) => {
+        this._stringFor(flagProperty, "insensitive", (attrValue, attrSpaces) => {
           if (
             attrValue.length > 0 &&
             !this.quoted &&