diff --git a/mgq.js b/mgq.js
index 45da8e2..e00b9ac 100644
--- a/mgq.js
+++ b/mgq.js
@@ -49,7 +49,7 @@ const condOps = new Set([
 ]);
 
 // Set of query operators
-const queryOps = new Set(["$and", "$or", "$nor"]);
+const queryOps = new Set(["$and", "$or", "$nor", "$where"]);
 
 /**
  * Creates a new Query object
@@ -97,6 +97,9 @@ function matchCond(query, doc) {
 			if (path === "$nor") {
 				results.push(matchNor(doc, path, query.$nor));
 			}
+			if (path === "$where") {
+				results.push(matchWhere(doc, path, query.$where));
+			}
 		} else {
 			const expOrOv = query[path];
 			const isAllExp = checkAllExp(expOrOv);
@@ -182,7 +185,11 @@ function validate(query) {
 					throw new TypeError("$nor operator value must be an array");
 				}
 			}
-
+			if (path === "$where" && !validateWhere(query.$where)) {
+				throw new TypeError(
+					"$where operator value must be a function or a string",
+				);
+			}
 			if (Array.isArray(query[path])) {
 				for (const cond of query[path]) {
 					validate(cond);
@@ -304,6 +311,15 @@ function validateSize(value) {
 	return typeof value === "number";
 }
 
+/**
+ * Validates $where operator
+ * @param {any} value - The value to validate
+ * @returns {boolean}
+ */
+function validateWhere(value) {
+	return typeof value === "function" || typeof value === "string";
+}
+
 /**
  * Checks if the given value is a plain object
  * (not null, not an array, not a regex, not a date)
@@ -1081,3 +1097,27 @@ function matchAll(doc, path, ov) {
 
 	return false;
 }
+
+/**
+ * Matches if the value at the given path matches the queried $where
+ * @param {any} doc - Document to check
+ * @param {string} path - Path to the value
+ * @param {any} ov - Value to match against
+ * @returns {boolean}
+ */
+function matchWhere(doc, path, ov) {
+	if (!validateWhere(ov)) {
+		return false;
+	}
+
+	if (typeof ov === "function") {
+		return ov.call(doc);
+	}
+
+	if (typeof ov === "string") {
+		const fn = new Function(ov);
+		return fn.call(doc);
+	}
+
+	return false;
+}
diff --git a/test/utils.js b/test/utils.js
index 975f8dc..6e341a6 100644
--- a/test/utils.js
+++ b/test/utils.js
@@ -10,9 +10,13 @@ import { Collection } from "mongodb";
 export async function getMongoResults(collection, query, input) {
 	try {
 		await collection.insertMany(structuredClone(input));
-		const results = await collection.find(query).project({ _id: 0 }).toArray();
+		const results = await collection
+			.find(query, { serializeFunctions: true })
+			.project({ _id: 0 })
+			.toArray();
 		return results;
 	} catch (error) {
+		console.error(error);
 		return [];
 	}
 }
diff --git a/test/validate.test.js b/test/validate.test.js
index d45b563..e5543dc 100644
--- a/test/validate.test.js
+++ b/test/validate.test.js
@@ -116,6 +116,13 @@ describe("Query Validation", () => {
 			TypeError,
 		);
 	});
+
+	test("$where validation", () => {
+		assert.doesNotThrow(() => Query({ $where: () => {} }).validate());
+		assert.doesNotThrow(() => Query({ $where: "return true" }).validate());
+		assert.throws(() => Query({ $where: {} }).validate(), TypeError);
+		assert.throws(() => Query({ $where: 123 }).validate(), TypeError);
+	});
 });
 
 test("validate should return query", () => {
diff --git a/test/where.test.js b/test/where.test.js
new file mode 100644
index 0000000..0d855f6
--- /dev/null
+++ b/test/where.test.js
@@ -0,0 +1,100 @@
+import assert from "node:assert";
+import test, { after, afterEach, before, describe } from "node:test";
+import { Query } from "../mgq.js";
+import { getFilterResults, getMongoResults } from "./utils.js";
+import { Collection, MongoClient } from "mongodb";
+import { MongoMemoryServer } from "mongodb-memory-server";
+
+const testCases = [
+	{
+		name: "$where string function",
+		query: {
+			$where: "return Boolean(this.foo) && this.foo.length === 2",
+		},
+		input: [
+			{ foo: "ab" },
+			{ foo: [1, "a"] },
+			{ foo: [{}, {}] },
+			{ foo: [1, 2, 3] },
+			{ foo: [] },
+			{ foo: null },
+		],
+		expected: [{ foo: "ab" }, { foo: [1, "a"] }, { foo: [{}, {}] }],
+	},
+	{
+		name: "$where declared function",
+		query: {
+			$where: function () {
+				return this.foo && this.foo.length === 2;
+			},
+		},
+		input: [
+			{ foo: "ab" },
+			{ foo: [1, "a"] },
+			{ foo: [{}, {}] },
+			{ foo: [1, 2, 3] },
+			{ foo: [] },
+			{ foo: null },
+		],
+		expected: [{ foo: "ab" }, { foo: [1, "a"] }, { foo: [{}, {}] }],
+	},
+	{
+		name: "$where invalid query",
+		query: {
+			$where: { foo: "bar" },
+		},
+		input: [{ foo: "bar" }],
+		expected: [],
+	},
+];
+
+/** @type {MongoMemoryServer} */
+let mongod;
+
+/** @type {MongoClient} */
+let client;
+
+/** @type {Collection} */
+let collection;
+
+before(async () => {
+	try {
+		mongod = await MongoMemoryServer.create();
+		const uri = mongod.getUri();
+		client = new MongoClient(uri);
+		await client.connect();
+		collection = client.db("test").collection("test");
+	} catch (error) {
+		console.error(error);
+	}
+});
+
+after(async () => {
+	try {
+		await client.close();
+		await mongod.stop();
+	} catch (error) {
+		console.error(error);
+	}
+});
+
+afterEach(async () => {
+	try {
+		await collection.deleteMany({});
+	} catch (error) {
+		console.error(error);
+	}
+});
+
+describe("Query $where tests", async () => {
+	for (const { name, query, input, expected } of testCases) {
+		await test(name, async () => {
+			const mongoExpected = await getMongoResults(collection, query, input);
+			assert.deepStrictEqual(mongoExpected, expected);
+
+			const q = new Query(query);
+			const actual = getFilterResults(q.test.bind(q), input);
+			assert.deepStrictEqual(actual, expected);
+		});
+	}
+});