diff --git a/README.md b/README.md
index 3ecdefc5..faa95c89 100644
--- a/README.md
+++ b/README.md
@@ -20,6 +20,7 @@
     * Segment Tree or Interval Tree
     * Binary Indexed Tree or Fenwick Tree
 9. [Graph](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/graph) (both directed and undirected)
+9. [Disjoint Set](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/disjoint-set)
 
 ## Algorithms
 
diff --git a/src/data-structures/disjoint-set/DisjointSet.js b/src/data-structures/disjoint-set/DisjointSet.js
new file mode 100644
index 00000000..c7f786c0
--- /dev/null
+++ b/src/data-structures/disjoint-set/DisjointSet.js
@@ -0,0 +1,93 @@
+import DisjointSetItem from './DisjointSetItem';
+
+export default class DisjointSet {
+  /**
+   * @param {function(value: *)} [keyCallback]
+   */
+  constructor(keyCallback) {
+    this.keyCallback = keyCallback;
+    this.items = {};
+  }
+
+  /**
+   * @param {*} itemValue
+   * @return {DisjointSet}
+   */
+  makeSet(itemValue) {
+    const disjointSetItem = new DisjointSetItem(itemValue, this.keyCallback);
+
+    if (!this.items[disjointSetItem.getKey()]) {
+      // Add new item only in case if it not presented yet.
+      this.items[disjointSetItem.getKey()] = disjointSetItem;
+    }
+
+    return this;
+  }
+
+  /**
+   * @param {*} itemValue
+   * @return {(string|null)}
+   */
+  find(itemValue) {
+    const templateDisjointItem = new DisjointSetItem(itemValue, this.keyCallback);
+
+    // Try to find item itself;
+    const requiredDisjointItem = this.items[templateDisjointItem.getKey()];
+
+    if (!requiredDisjointItem) {
+      return null;
+    }
+
+    return requiredDisjointItem.getRoot().getKey();
+  }
+
+  /**
+   * @param {*} valueA
+   * @param {*} valueB
+   * @return {DisjointSet}
+   */
+  union(valueA, valueB) {
+    const rootKeyA = this.find(valueA);
+    const rootKeyB = this.find(valueB);
+
+    if (rootKeyA === null || rootKeyB === null) {
+      throw new Error('One or two values are not in sets');
+    }
+
+    if (rootKeyA === rootKeyB) {
+      // In case if both elements are already in the same set then just return its key.
+      return this;
+    }
+
+    const rootA = this.items[rootKeyA];
+    const rootB = this.items[rootKeyB];
+
+    if (rootA.getAncestorsCount() < rootB.getAncestorsCount()) {
+      // If rootB's tree is bigger then make rootB to be a new root.
+      rootB.addChild(rootA);
+
+      return rootB.getKey();
+    }
+
+    // If rootA's tree is bigger then make rootA to be a new root.
+    rootA.addChild(rootB);
+
+    return this;
+  }
+
+  /**
+   * @param {*} valueA
+   * @param {*} valueB
+   * @return {boolean}
+   */
+  inSameSet(valueA, valueB) {
+    const rootKeyA = this.find(valueA);
+    const rootKeyB = this.find(valueB);
+
+    if (rootKeyA === null || rootKeyB === null) {
+      throw new Error('One or two values are not in sets');
+    }
+
+    return rootKeyA === rootKeyB;
+  }
+}
diff --git a/src/data-structures/disjoint-set/DisjointSetItem.js b/src/data-structures/disjoint-set/DisjointSetItem.js
new file mode 100644
index 00000000..fece064f
--- /dev/null
+++ b/src/data-structures/disjoint-set/DisjointSetItem.js
@@ -0,0 +1,94 @@
+export default class DisjointSetItem {
+  /**
+   * @param {*} value
+   * @param {function(value: *)} [keyCallback]
+   */
+  constructor(value, keyCallback) {
+    this.value = value;
+    this.keyCallback = keyCallback;
+    /** @var {DisjointSetItem} this.parent */
+    this.parent = null;
+    this.children = {};
+  }
+
+  /**
+   * @return {*}
+   */
+  getKey() {
+    // Allow user to define custom key generator.
+    if (this.keyCallback) {
+      return this.keyCallback(this.value);
+    }
+
+    // Otherwise use value as a key by default.
+    return this.value;
+  }
+
+  /**
+   * @return {DisjointSetItem}
+   */
+  getRoot() {
+    return this.isRoot() ? this : this.parent.getRoot();
+  }
+
+  /**
+   * @return {boolean}
+   */
+  isRoot() {
+    return this.parent === null;
+  }
+
+  /**
+   * @return {number}
+   */
+  getAncestorsCount() {
+    if (this.getChildren().length === 0) {
+      return 0;
+    }
+
+    let count = 0;
+
+    /** @var {DisjointSetItem} child */
+    this.getChildren().forEach((child) => {
+      // Count child itself.
+      count += 1;
+
+      // Also add all children of current child.
+      count += child.getAncestorsCount();
+    });
+
+    return count;
+  }
+
+  /**
+   * @return {DisjointSetItem[]}
+   */
+  getChildren() {
+    return Object.values(this.children);
+  }
+
+  /**
+   * @param {DisjointSetItem} parentItem
+   * @param {boolean} forceSettingParentChild
+   * @return {DisjointSetItem}
+   */
+  setParent(parentItem, forceSettingParentChild = true) {
+    this.parent = parentItem;
+    if (forceSettingParentChild) {
+      parentItem.addChild(this);
+    }
+
+    return this;
+  }
+
+  /**
+   * @param {DisjointSetItem} childItem
+   * @return {DisjointSetItem}
+   */
+  addChild(childItem) {
+    this.children[childItem.getKey()] = childItem;
+    childItem.setParent(this, false);
+
+    return this;
+  }
+}
diff --git a/src/data-structures/disjoint-set/README.md b/src/data-structures/disjoint-set/README.md
new file mode 100644
index 00000000..b5d5bfcd
--- /dev/null
+++ b/src/data-structures/disjoint-set/README.md
@@ -0,0 +1,20 @@
+# Disjoint Set
+
+**Disjoint-set** data structure (also called a union–find data structure or merge–find set) is a data 
+structure that tracks a set of elements partitioned into a number of disjoint (non-overlapping) subsets. 
+It provides near-constant-time operations (bounded by the inverse Ackermann function) to *add new sets*, 
+to *merge existing sets*, and to *determine whether elements are in the same set*. 
+In addition to many other uses (see the Applications section), disjoint-sets play a key role in Kruskal's algorithm for finding the minimum spanning tree of a graph.
+
+![disjoint set](https://upload.wikimedia.org/wikipedia/commons/6/67/Dsu_disjoint_sets_init.svg)
+
+*MakeSet* creates 8 singletons.
+
+![disjoint set](https://upload.wikimedia.org/wikipedia/commons/a/ac/Dsu_disjoint_sets_final.svg)
+
+After some operations of *Union*, some sets are grouped together.
+
+## References
+
+- [Wikipedia](https://en.wikipedia.org/wiki/Disjoint-set_data_structure)
+- [By Abdul Bari on YouTube](https://www.youtube.com/watch?v=wU6udHRIkcc)
diff --git a/src/data-structures/disjoint-set/__test__/DisjointSet.test.js b/src/data-structures/disjoint-set/__test__/DisjointSet.test.js
new file mode 100644
index 00000000..c62e6393
--- /dev/null
+++ b/src/data-structures/disjoint-set/__test__/DisjointSet.test.js
@@ -0,0 +1,140 @@
+import DisjointSet from '../DisjointSet';
+
+describe('DisjointSet', () => {
+  it('should throw error when trying to union and check not existing sets', () => {
+    function mergeNotExistingSets() {
+      const disjointSet = new DisjointSet();
+
+      disjointSet.union('A', 'B');
+    }
+
+    function checkNotExistingSets() {
+      const disjointSet = new DisjointSet();
+
+      disjointSet.inSameSet('A', 'B');
+    }
+
+    expect(mergeNotExistingSets).toThrow();
+    expect(checkNotExistingSets).toThrow();
+  });
+
+  it('should do basic manipulations on disjoint set', () => {
+    const disjointSet = new DisjointSet();
+
+    expect(disjointSet.find('A')).toBeNull();
+    expect(disjointSet.find('B')).toBeNull();
+
+    disjointSet.makeSet('A');
+
+    expect(disjointSet.find('A')).toBe('A');
+    expect(disjointSet.find('B')).toBeNull();
+
+    disjointSet.makeSet('B');
+
+    expect(disjointSet.find('A')).toBe('A');
+    expect(disjointSet.find('B')).toBe('B');
+
+    disjointSet.makeSet('C');
+
+    expect(disjointSet.inSameSet('A', 'B')).toBeFalsy();
+
+    disjointSet.union('A', 'B');
+
+    expect(disjointSet.find('A')).toBe('A');
+    expect(disjointSet.find('B')).toBe('A');
+    expect(disjointSet.inSameSet('A', 'B')).toBeTruthy();
+    expect(disjointSet.inSameSet('B', 'A')).toBeTruthy();
+    expect(disjointSet.inSameSet('A', 'C')).toBeFalsy();
+
+    disjointSet.union('A', 'A');
+
+    disjointSet.union('B', 'C');
+
+    expect(disjointSet.find('A')).toBe('A');
+    expect(disjointSet.find('B')).toBe('A');
+    expect(disjointSet.find('C')).toBe('A');
+
+    expect(disjointSet.inSameSet('A', 'B')).toBeTruthy();
+    expect(disjointSet.inSameSet('B', 'C')).toBeTruthy();
+    expect(disjointSet.inSameSet('A', 'C')).toBeTruthy();
+
+    disjointSet
+      .makeSet('E')
+      .makeSet('F')
+      .makeSet('G')
+      .makeSet('H')
+      .makeSet('I');
+
+    disjointSet
+      .union('E', 'F')
+      .union('F', 'G')
+      .union('G', 'H')
+      .union('H', 'I');
+
+    expect(disjointSet.inSameSet('A', 'I')).toBeFalsy();
+    expect(disjointSet.inSameSet('E', 'I')).toBeTruthy();
+
+    disjointSet.union('I', 'C');
+
+    expect(disjointSet.find('I')).toBe('E');
+    expect(disjointSet.inSameSet('A', 'I')).toBeTruthy();
+  });
+
+  it('should union smaller set with bigger one making bigger one to be new root', () => {
+    const disjointSet = new DisjointSet();
+
+    disjointSet
+      .makeSet('A')
+      .makeSet('B')
+      .makeSet('C')
+      .union('B', 'C')
+      .union('A', 'C');
+
+    expect(disjointSet.find('A')).toBe('B');
+  });
+
+  it('should do basic manipulations on disjoint set with custom key extractor', () => {
+    const keyExtractor = value => value.key;
+
+    const disjointSet = new DisjointSet(keyExtractor);
+
+    const itemA = { key: 'A', value: 1 };
+    const itemB = { key: 'B', value: 2 };
+    const itemC = { key: 'C', value: 3 };
+
+    expect(disjointSet.find(itemA)).toBeNull();
+    expect(disjointSet.find(itemB)).toBeNull();
+
+    disjointSet.makeSet(itemA);
+
+    expect(disjointSet.find(itemA)).toBe('A');
+    expect(disjointSet.find(itemB)).toBeNull();
+
+    disjointSet.makeSet(itemB);
+
+    expect(disjointSet.find(itemA)).toBe('A');
+    expect(disjointSet.find(itemB)).toBe('B');
+
+    disjointSet.makeSet(itemC);
+
+    expect(disjointSet.inSameSet(itemA, itemB)).toBeFalsy();
+
+    disjointSet.union(itemA, itemB);
+
+    expect(disjointSet.find(itemA)).toBe('A');
+    expect(disjointSet.find(itemB)).toBe('A');
+    expect(disjointSet.inSameSet(itemA, itemB)).toBeTruthy();
+    expect(disjointSet.inSameSet(itemB, itemA)).toBeTruthy();
+    expect(disjointSet.inSameSet(itemA, itemC)).toBeFalsy();
+
+    disjointSet.union(itemA, itemC);
+
+    expect(disjointSet.find(itemA)).toBe('A');
+    expect(disjointSet.find(itemB)).toBe('A');
+    expect(disjointSet.find(itemC)).toBe('A');
+
+    expect(disjointSet.inSameSet(itemA, itemB)).toBeTruthy();
+    expect(disjointSet.inSameSet(itemB, itemC)).toBeTruthy();
+    expect(disjointSet.inSameSet(itemA, itemC)).toBeTruthy();
+  });
+});
diff --git a/src/data-structures/disjoint-set/__test__/DisjointSetItem.test.js b/src/data-structures/disjoint-set/__test__/DisjointSetItem.test.js
new file mode 100644
index 00000000..c3f1000d
--- /dev/null
+++ b/src/data-structures/disjoint-set/__test__/DisjointSetItem.test.js
@@ -0,0 +1,115 @@
+import DisjointSetItem from '../DisjointSetItem';
+
+describe('DisjointSetItem', () => {
+  it('should do basic manipulation with disjoint set item', () => {
+    const itemA = new DisjointSetItem('A');
+    const itemB = new DisjointSetItem('B');
+    const itemC = new DisjointSetItem('C');
+    const itemD = new DisjointSetItem('D');
+
+    expect(itemA.getAncestorsCount()).toBe(0);
+    expect(itemA.getChildren()).toEqual([]);
+    expect(itemA.getKey()).toBe('A');
+    expect(itemA.getRoot()).toEqual(itemA);
+    expect(itemA.isRoot()).toBeTruthy();
+    expect(itemB.isRoot()).toBeTruthy();
+
+    itemA.addChild(itemB);
+    itemD.setParent(itemC);
+
+    expect(itemA.getAncestorsCount()).toBe(1);
+    expect(itemC.getAncestorsCount()).toBe(1);
+
+    expect(itemB.getAncestorsCount()).toBe(0);
+    expect(itemD.getAncestorsCount()).toBe(0);
+
+    expect(itemA.getChildren().length).toBe(1);
+    expect(itemC.getChildren().length).toBe(1);
+
+    expect(itemA.getChildren()[0]).toEqual(itemB);
+    expect(itemC.getChildren()[0]).toEqual(itemD);
+
+    expect(itemB.getChildren().length).toBe(0);
+    expect(itemD.getChildren().length).toBe(0);
+
+    expect(itemA.getRoot()).toEqual(itemA);
+    expect(itemB.getRoot()).toEqual(itemA);
+
+    expect(itemC.getRoot()).toEqual(itemC);
+    expect(itemD.getRoot()).toEqual(itemC);
+
+    expect(itemA.isRoot()).toBeTruthy();
+    expect(itemB.isRoot()).toBeFalsy();
+    expect(itemC.isRoot()).toBeTruthy();
+    expect(itemD.isRoot()).toBeFalsy();
+
+    itemA.addChild(itemC);
+
+    expect(itemA.isRoot()).toBeTruthy();
+    expect(itemB.isRoot()).toBeFalsy();
+    expect(itemC.isRoot()).toBeFalsy();
+    expect(itemD.isRoot()).toBeFalsy();
+
+    expect(itemA.getAncestorsCount()).toEqual(3);
+    expect(itemB.getAncestorsCount()).toEqual(0);
+    expect(itemC.getAncestorsCount()).toEqual(1);
+  });
+
+  it('should do basic manipulation with disjoint set item with custom key extractor', () => {
+    const keyExtractor = (value) => {
+      return value.key;
+    };
+
+    const itemA = new DisjointSetItem({ key: 'A', value: 1 }, keyExtractor);
+    const itemB = new DisjointSetItem({ key: 'B', value: 2 }, keyExtractor);
+    const itemC = new DisjointSetItem({ key: 'C', value: 3 }, keyExtractor);
+    const itemD = new DisjointSetItem({ key: 'D', value: 4 }, keyExtractor);
+
+    expect(itemA.getAncestorsCount()).toBe(0);
+    expect(itemA.getChildren()).toEqual([]);
+    expect(itemA.getKey()).toBe('A');
+    expect(itemA.getRoot()).toEqual(itemA);
+    expect(itemA.isRoot()).toBeTruthy();
+    expect(itemB.isRoot()).toBeTruthy();
+
+    itemA.addChild(itemB);
+    itemD.setParent(itemC);
+
+    expect(itemA.getAncestorsCount()).toBe(1);
+    expect(itemC.getAncestorsCount()).toBe(1);
+
+    expect(itemB.getAncestorsCount()).toBe(0);
+    expect(itemD.getAncestorsCount()).toBe(0);
+
+    expect(itemA.getChildren().length).toBe(1);
+    expect(itemC.getChildren().length).toBe(1);
+
+    expect(itemA.getChildren()[0]).toEqual(itemB);
+    expect(itemC.getChildren()[0]).toEqual(itemD);
+
+    expect(itemB.getChildren().length).toBe(0);
+    expect(itemD.getChildren().length).toBe(0);
+
+    expect(itemA.getRoot()).toEqual(itemA);
+    expect(itemB.getRoot()).toEqual(itemA);
+
+    expect(itemC.getRoot()).toEqual(itemC);
+    expect(itemD.getRoot()).toEqual(itemC);
+
+    expect(itemA.isRoot()).toBeTruthy();
+    expect(itemB.isRoot()).toBeFalsy();
+    expect(itemC.isRoot()).toBeTruthy();
+    expect(itemD.isRoot()).toBeFalsy();
+
+    itemA.addChild(itemC);
+
+    expect(itemA.isRoot()).toBeTruthy();
+    expect(itemB.isRoot()).toBeFalsy();
+    expect(itemC.isRoot()).toBeFalsy();
+    expect(itemD.isRoot()).toBeFalsy();
+
+    expect(itemA.getAncestorsCount()).toEqual(3);
+    expect(itemB.getAncestorsCount()).toEqual(0);
+    expect(itemC.getAncestorsCount()).toEqual(1);
+  });
+});