diff --git a/.changeset/cute-chairs-kneel.md b/.changeset/cute-chairs-kneel.md
new file mode 100644
index 00000000000..e00e03eced1
--- /dev/null
+++ b/.changeset/cute-chairs-kneel.md
@@ -0,0 +1,5 @@
+---
+"effect": minor
+---
+
+Add Function.memo
diff --git a/packages/effect/src/Function.ts b/packages/effect/src/Function.ts
index 69d53fd2fdd..03393376ecc 100644
--- a/packages/effect/src/Function.ts
+++ b/packages/effect/src/Function.ts
@@ -1220,3 +1220,194 @@ export const hole: <T>() => T = unsafeCoerce(absurd)
  * @since 2.0.0
  */
 export const SK = <A, B>(_: A, b: B): B => b
+
+/**
+ * Empty ArgNode result sentinel.
+ *
+ * @internal
+ */
+const EMPTY = Symbol.for("effect/Function.memo")
+
+/**
+ * Memo helper class.
+ *
+ * @internal
+ */
+class ArgNode<A> {
+  /** The cached result of the args that led to this node, or `EMPTY`. */
+  r: A | typeof EMPTY = EMPTY
+
+  /** The primitive arg branch. */
+  private p: Map<unknown, ArgNode<A>> | null = null
+
+  /** The object arg branch. */
+  private o: WeakMap<object, ArgNode<A>> | null = null
+
+  /** Get the next node for an arg. If uninitialized, one will be created, set, and returned. */
+  at(arg: unknown): ArgNode<A> {
+    let next: ArgNode<A> | undefined
+    let isObject: boolean
+    switch (typeof arg) {
+      case "object":
+      // @ts-expect-error Fallthrough intended for primitive null case
+      case "function":
+        if (arg !== null) {
+          if (!this.o) this.o = new WeakMap()
+          next = this.o.get(arg)
+          isObject = true
+          break
+        }
+
+      default:
+        if (!this.p) this.p = new Map()
+        next = this.p.get(arg)
+        isObject = false
+    }
+
+    if (next) return next
+
+    const fresh = new ArgNode<A>()
+
+    if (isObject) {
+      this.o!.set(arg as object, fresh)
+    } else {
+      this.p!.set(arg, fresh)
+    }
+
+    return fresh
+  }
+}
+
+/**
+ * A global WeakMap from original functions to memo wrappers, so that we can avoid re-wrapping.
+ *
+ * @internal
+ */
+const origToMemo = new WeakMap<Function, Function>()
+/**
+ * Private branding for memo wrappers.
+ *
+ * @internal
+ */
+const allMemos = new WeakSet<Function>()
+
+export declare namespace memo {
+  interface Options {
+    /**
+     * Optimizes caching by not setting the path for trailing `undefined`
+     * arguments. This is useful for functions with optional arguments, treating
+     * provided-`undefined` and absent-`undefined` the same way. The wrapped
+     * function will nevertheless receive all arguments.
+     *
+     * This option should be disabled if:
+     *
+     * - The function has variadic arguments and `undefined` is a valid input
+     * - The function distinguishes `f(1)` vs `f(1, undefined)` via `arguments.length` or `rest.length`
+     *
+     * @default true
+     */
+    readonly trimUndefined?: boolean
+  }
+}
+
+/**
+ * Memoize a function, method, or getter, with any number of arguments. Repeated calls to the returned function with the same arguments will return cached values.
+ *
+ * Usage notes:
+ * - Memoized functions should be totally pure, and should return immutable values.
+ * - The cache size is unbounded, but internally a `WeakMap` is used when possible. To make the most of this, memoized functions should have object-type args at the start and primitive args at the end.
+ * - Works as a class method decorator under modern settings (`experimentalDecorators: false`), though you will have to use the curried form (`@Function.memo()`).
+ *
+ * @example
+ * ```ts
+ * import { memo } from "effect/Function"
+ *
+ * const add = memo((x: number, y: number) => {
+ *   console.log("running add");
+ *   return x + y;
+ * });
+ *
+ * add(2, 3); // logs "running add", returns 5
+ * add(2, 3); // no log, returns cached 5
+ * add(2, 4); // logs "running add", returns 6
+ *
+ * // Expected console output:
+ * // running add
+ * // running add
+ * ```
+ *
+ * @since 3.20.0
+ */
+export const memo: {
+  (
+    options?: memo.Options
+  ): <Args extends Array<any>, Return>(fn: (...args: Args) => Return) => (...args: Args) => Return
+  <Args extends Array<any>, Return>(
+    fn: (...args: Args) => Return,
+    options?: memo.Options
+  ): (...args: Args) => Return
+} = dual(
+  (args) => isFunction(args[0]),
+  <Args extends Array<any>, Return>(
+    fn: (...args: Args) => Return,
+    options?: memo.Options
+  ): (...args: Args) => Return => {
+    // If input is a 'base' function and already memoized, return it
+    if (origToMemo.has(fn)) return origToMemo.get(fn) as any
+
+    // If input is a memo function, don't re-wrap it
+    if (allMemos.has(fn)) return fn
+
+    // Create the `ArgNode` root for this function
+    const root = new ArgNode<Return>()
+
+    const shouldTrim = options?.trimUndefined ?? true
+
+    const out = function(this: unknown, ...args: Args): Return {
+      // Find the defined length
+      let argsLength = args.length
+      if (shouldTrim) {
+        let definedLength = argsLength
+        while (definedLength > 0 && args[definedLength - 1] === undefined) {
+          definedLength -= 1
+        }
+        if (definedLength < argsLength) {
+          argsLength = definedLength
+        }
+      }
+
+      // Drill through `this` and `args` to get the ArgNode that holds the result
+      let node = root.at(this)
+      for (let i = 0; i < argsLength; i += 1) {
+        node = node.at(args[i])
+      }
+
+      if (node.r !== EMPTY) return node.r
+
+      const result = fn.apply(this, args)
+      node.r = result
+
+      return result
+    }
+
+    origToMemo.set(fn, out)
+    allMemos.add(out)
+
+    return out
+  }
+)
+
+/**
+ * See {@link memo}. This is an alias that infers `This` and uses it in the returned function signature.
+ *
+ * @since 3.20.0
+ */
+export const memoThis: {
+  (options?: memo.Options): <This, Args extends Array<any>, Return>(
+    fn: (this: This, ...args: Args) => Return
+  ) => (this: This, ...args: Args) => Return
+  <This, Args extends Array<any>, Return>(
+    fn: (this: This, ...args: Args) => Return,
+    options?: memo.Options
+  ): (this: This, ...args: Args) => Return
+} = memo
diff --git a/packages/effect/test/Function.test.ts b/packages/effect/test/Function.test.ts
index de67d89dc5a..8e08a1ca0f2 100644
--- a/packages/effect/test/Function.test.ts
+++ b/packages/effect/test/Function.test.ts
@@ -1,4 +1,4 @@
-import { describe, it } from "@effect/vitest"
+import { describe, expect, it, vi } from "@effect/vitest"
 import { deepStrictEqual, strictEqual, throws } from "@effect/vitest/utils"
 import { Function, String } from "effect"
 
@@ -195,4 +195,132 @@ describe("Function", () => {
       deepStrictEqual(f.apply(null, ["_", "a", "b", "c", "d", "e", "f"] as any), "_abcde")
     })
   })
+
+  describe("memo", () => {
+    it("memoizes by primitive argument list", () => {
+      const fn = vi.fn((a: number, b: number) => a + b)
+      const memoFn = Function.memo(fn)
+
+      strictEqual(memoFn(1, 2), 3)
+      expect(fn).toHaveBeenCalledTimes(1)
+
+      // Cache hit (same args)
+      strictEqual(memoFn(1, 2), 3)
+      expect(fn).toHaveBeenCalledTimes(1)
+
+      // Different args -> cache miss
+      strictEqual(memoFn(2, 1), 3)
+      expect(fn).toHaveBeenCalledTimes(2)
+    })
+
+    it("memoizes zero-argument functions and shares cache per fn", () => {
+      const fn = vi.fn(() => 42)
+      const memoFnA = Function.memo(fn)
+      const memoFnB = Function.memo(fn) // wrappers share same per-fn cache
+
+      expect(memoFnA()).toBe(42)
+      expect(fn).toHaveBeenCalledTimes(1)
+
+      // Cache hit via same wrapper
+      expect(memoFnA()).toBe(42)
+      expect(fn).toHaveBeenCalledTimes(1)
+
+      // Cache hit via different wrapper for same original function
+      expect(memoFnB()).toBe(42)
+      expect(fn).toHaveBeenCalledTimes(1)
+    })
+
+    it("trims trailing undefined optional args by default", () => {
+      const fn = vi.fn(function(a?: number, b?: number) {
+        return { a, b, length: arguments.length }
+      })
+      const m = Function.memo(fn)
+
+      expect(m(1)).toEqual({ a: 1, b: undefined, length: 1 })
+      expect(fn).toHaveBeenCalledTimes(1)
+
+      expect(m(1, undefined)).toEqual({ a: 1, b: undefined, length: 1 })
+      expect(fn).toHaveBeenCalledTimes(1)
+
+      expect(m(1, 2)).toEqual({ a: 1, b: 2, length: 2 })
+      expect(fn).toHaveBeenCalledTimes(2)
+    })
+
+    it("preserves trailing undefined when trimUndefined is disabled", () => {
+      const fn = vi.fn(function(a?: number, b?: number) {
+        return { a, b, length: arguments.length }
+      })
+      const m = Function.memo(fn, { trimUndefined: false })
+
+      expect(m(1)).toEqual({ a: 1, b: undefined, length: 1 })
+      expect(fn).toHaveBeenCalledTimes(1)
+
+      expect(m(1, undefined)).toEqual({ a: 1, b: undefined, length: 2 })
+      expect(fn).toHaveBeenCalledTimes(2)
+
+      expect(m(1)).toEqual({ a: 1, b: undefined, length: 1 })
+      expect(fn).toHaveBeenCalledTimes(2)
+    })
+
+    it("only trims trailing undefined values", () => {
+      const fn = vi.fn((a?: number, b?: number, c?: number) => [a, b, c])
+      const m = Function.memo(fn)
+
+      expect(m(1, undefined, 3)).toEqual([1, undefined, 3])
+      expect(fn).toHaveBeenCalledTimes(1)
+
+      expect(m(1, 3)).toEqual([1, 3, undefined])
+      expect(fn).toHaveBeenCalledTimes(2)
+    })
+
+    it("memoizes undefined results distinctly", () => {
+      const fn = vi.fn((_: number): void => undefined)
+      const memoFn = Function.memo(fn)
+
+      expect(memoFn(123)).toBeUndefined()
+      expect(fn).toHaveBeenCalledTimes(1)
+
+      // Cache hit despite undefined result
+      expect(memoFn(123)).toBeUndefined()
+      expect(fn).toHaveBeenCalledTimes(1)
+    })
+
+    it("uses object identity for object args", () => {
+      const a = { x: 1 }
+      const b = { x: 1 }
+
+      const fn = vi.fn((o: { x: number }) => o.x)
+      const memoFn = Function.memo(fn)
+
+      expect(memoFn(a)).toBe(1)
+      expect(fn).toHaveBeenCalledTimes(1)
+
+      // Cache hit for same object instance
+      expect(memoFn(a)).toBe(1)
+      expect(fn).toHaveBeenCalledTimes(1)
+
+      // Different object instance with same shape -> cache miss
+      expect(memoFn(b)).toBe(1)
+      expect(fn).toHaveBeenCalledTimes(2)
+    })
+
+    it("includes receiver (this) in the cache key", () => {
+      function plusOffset(this: { offset: number }, x: number) {
+        return x + this.offset
+      }
+      const fnThis = vi.fn(plusOffset)
+      // Casts only needed to appease TS 'this' typing for test invocations
+      const memoFn = Function.memo(fnThis)
+
+      const a = { offset: 1 }
+      const b = { offset: 100 }
+
+      expect(memoFn.call(a, 1)).toBe(2)
+      expect(fnThis).toHaveBeenCalledTimes(1)
+
+      // Different receiver with same args should not reuse prior result
+      expect(memoFn.call(b, 1)).toBe(101)
+      expect(fnThis).toHaveBeenCalledTimes(2)
+    })
+  })
 })