diff --git a/ROADMAP.md b/ROADMAP.md
index d20bd3d..391840b 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -110,7 +110,7 @@
 **Compression & encoding**
   - [x] Run-length encoding (RLE)
   - [x] Huffman coding utilities
-  - [ ] LZ77 dictionary compression helper
+  - [x] LZ77 dictionary compression helper
   - [x] Base64 encode/decode utilities
 **Geometric & numeric utilities**
   - [ ] Closest pair of points solver for geometry toolkit
diff --git a/docs/index.d.ts b/docs/index.d.ts
index 17026d3..7b35926 100644
--- a/docs/index.d.ts
+++ b/docs/index.d.ts
@@ -112,6 +112,8 @@ export const examples: {
     readonly createHuffmanTable: 'examples/huffman.ts';
     readonly huffmanEncode: 'examples/huffman.ts';
     readonly huffmanDecode: 'examples/huffman.ts';
+    readonly lz77Compress: 'examples/lz77.ts';
+    readonly lz77Decompress: 'examples/lz77.ts';
   };
   readonly performance: {
     readonly debounce: 'examples/requestDedup.ts';
@@ -3086,6 +3088,16 @@ export function createHuffmanTable(input: string): HuffmanTable;
 export function huffmanEncode(input: string): HuffmanEncodedResult;
 export function huffmanDecode(encoded: string, table: Readonly<HuffmanTable>): string;
 
+/**
+ * LZ77 dictionary compression helper.
+ * Use for: streaming compression with sliding window back-references.
+ * Import: data/lz77.ts
+ */
+export interface Lz77Token { offset: number; length: number; next: string }
+export interface Lz77Options { windowSize?: number; lookaheadSize?: number }
+export function lz77Compress(input: string, options?: Lz77Options): Lz77Token[];
+export function lz77Decompress(tokens: ReadonlyArray<Lz77Token>): string;
+
 /**
  * Disjoint Set Union (Union-Find) with path compression and union by size.
  * Use for: connectivity queries, Kruskal MST, clustering.
diff --git a/examples/lz77.ts b/examples/lz77.ts
new file mode 100644
index 0000000..4f8be8a
--- /dev/null
+++ b/examples/lz77.ts
@@ -0,0 +1,6 @@
+import { lz77Compress, lz77Decompress } from '../src/index.js';
+
+const input = 'abracadabra abracadabra';
+const tokens = lz77Compress(input, { windowSize: 12, lookaheadSize: 8 });
+console.log(tokens);
+console.log(lz77Decompress(tokens));
diff --git a/src/data/lz77.ts b/src/data/lz77.ts
new file mode 100644
index 0000000..7335d0f
--- /dev/null
+++ b/src/data/lz77.ts
@@ -0,0 +1,83 @@
+export interface Lz77Token {
+  offset: number;
+  length: number;
+  next: string;
+}
+
+export interface Lz77Options {
+  windowSize?: number;
+  lookaheadSize?: number;
+}
+
+export function lz77Compress(input: string, options: Lz77Options = {}): Lz77Token[] {
+  if (typeof input !== 'string') {
+    throw new TypeError('input must be a string.');
+  }
+  const windowSize = Math.max(1, Math.floor(options.windowSize ?? 32));
+  const lookaheadSize = Math.max(1, Math.floor(options.lookaheadSize ?? 16));
+  const tokens: Lz77Token[] = [];
+  let position = 0;
+
+  while (position < input.length) {
+    const windowStart = Math.max(0, position - windowSize);
+    const window = input.slice(windowStart, position);
+    let bestOffset = 0;
+    let bestLength = 0;
+
+    for (let offset = 1; offset <= window.length; offset += 1) {
+      let matchLength = 0;
+      while (
+        matchLength < lookaheadSize &&
+        position + matchLength < input.length &&
+        window[window.length - offset + (matchLength % offset)] === input[position + matchLength]
+      ) {
+        matchLength += 1;
+      }
+      if (matchLength > bestLength) {
+        bestLength = matchLength;
+        bestOffset = offset;
+      }
+    }
+
+    const nextChar = input[position + bestLength] ?? '';
+    tokens.push({ offset: bestOffset, length: bestLength, next: nextChar });
+    position += bestLength + 1;
+  }
+
+  return tokens;
+}
+
+export function lz77Decompress(tokens: ReadonlyArray<Lz77Token>): string {
+  let output = '';
+  for (const token of tokens) {
+    if (!token) continue;
+    const { offset, length, next } = token;
+    if (offset < 0 || length < 0) {
+      throw new Error('offset and length must be non-negative.');
+    }
+    if (offset === 0 || length === 0) {
+      if (next) {
+        output += next;
+      }
+      continue;
+    }
+    if (offset > output.length) {
+      throw new Error('offset exceeds output length.');
+    }
+    let copied = '';
+    for (let i = 0; i < length; i += 1) {
+      const char = output[output.length - offset + (i % offset)];
+      if (char === undefined) {
+        throw new Error('Invalid offset/length combination.');
+      }
+      copied += char;
+    }
+    output += copied;
+    if (next) {
+      output += next;
+    }
+  }
+  return output;
+}
+
+export const __internals = { lz77Compress, lz77Decompress };
diff --git a/src/index.ts b/src/index.ts
index 49dd21c..428acf2 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -110,6 +110,8 @@ export const examples = {
     createHuffmanTable: 'examples/huffman.ts',
     huffmanEncode: 'examples/huffman.ts',
     huffmanDecode: 'examples/huffman.ts',
+    lz77Compress: 'examples/lz77.ts',
+    lz77Decompress: 'examples/lz77.ts',
   },
   performance: {
     debounce: 'examples/requestDedup.ts',
@@ -1096,6 +1098,13 @@ export { base64Encode, base64Decode } from './data/base64.js';
  */
 export { createHuffmanTable, huffmanEncode, huffmanDecode } from './data/huffman.js';
 export type { HuffmanTable, HuffmanEncodedResult } from './data/huffman.js';
+/**
+ * LZ77 dictionary compression helpers for sliding window encoding.
+ *
+ * Example file: examples/lz77.ts
+ */
+export { lz77Compress, lz77Decompress } from './data/lz77.js';
+export type { Lz77Token, Lz77Options } from './data/lz77.js';
 
 export type {
   TreeNode,
diff --git a/tests/index.test.ts b/tests/index.test.ts
index 4672365..e170ffc 100644
--- a/tests/index.test.ts
+++ b/tests/index.test.ts
@@ -30,6 +30,8 @@ describe('package entry point', () => {
     expect(examples.data.createHuffmanTable).toBe('examples/huffman.ts');
     expect(examples.data.huffmanEncode).toBe('examples/huffman.ts');
     expect(examples.data.huffmanDecode).toBe('examples/huffman.ts');
+    expect(examples.data.lz77Compress).toBe('examples/lz77.ts');
+    expect(examples.data.lz77Decompress).toBe('examples/lz77.ts');
     expect(examples.visual.hexToRgb).toBe('examples/color.ts');
     expect(examples.visual.rgbToHex).toBe('examples/color.ts');
     expect(examples.visual.rgbToHsl).toBe('examples/color.ts');
@@ -142,6 +144,8 @@ describe('package entry point', () => {
       | 'createHuffmanTable'
       | 'huffmanEncode'
       | 'huffmanDecode'
+      | 'lz77Compress'
+      | 'lz77Decompress'
     >();
 
     expectTypeOf<ExampleName<'search'>>().toEqualTypeOf<
diff --git a/tests/lz77.test.ts b/tests/lz77.test.ts
new file mode 100644
index 0000000..0231e22
--- /dev/null
+++ b/tests/lz77.test.ts
@@ -0,0 +1,27 @@
+import { describe, expect, it } from 'vitest';
+
+import { lz77Compress, lz77Decompress } from '../src/data/lz77.js';
+
+describe('LZ77 compression', () => {
+  it('round-trips a string with repeated sequences', () => {
+    const input = 'abracadabra abracadabra';
+    const tokens = lz77Compress(input, { windowSize: 12, lookaheadSize: 8 });
+    const decoded = lz77Decompress(tokens);
+    expect(decoded).toBe(input);
+  });
+
+  it('compresses highly repetitive data', () => {
+    const input = 'aaaaaaaaaaaaaaaabbbbbbbbbbbb';
+    const tokens = lz77Compress(input, { windowSize: 16 });
+    expect(tokens.length).toBeLessThan(input.length);
+    expect(lz77Decompress(tokens)).toBe(input);
+  });
+
+  it('validates tokens during decompression', () => {
+    expect(() =>
+      lz77Decompress([
+        { offset: 5, length: 2, next: 'a' },
+      ])
+    ).toThrow('offset exceeds');
+  });
+});