diff --git a/src/sdam/topology.ts b/src/sdam/topology.ts
index 9f91d60f658..c4d81bd3ba8 100644
--- a/src/sdam/topology.ts
+++ b/src/sdam/topology.ts
@@ -52,6 +52,7 @@ import type { CloseOptions } from '../cmap/connection_pool';
 import { DestroyOptions, Connection } from '../cmap/connection';
 import type { MongoClientOptions } from '../mongo_client';
 import { DEFAULT_OPTIONS } from '../connection_string';
+import { serialize, deserialize } from '../bson';
 
 // Global state
 let globalTopologyCounter = 0;
@@ -188,6 +189,15 @@ export class Topology extends EventEmitter {
   static readonly OPEN = 'open' as const;
   /** @event */
   static readonly CONNECT = 'connect' as const;
+  /**
+   * @internal
+   *
+   * @privateRemarks
+   * mongodb-client-encryption's class ClientEncryption falls back to finding the bson lib
+   * defined on client.topology.bson, in order to maintain compatibility with any version
+   * of mongodb-client-encryption we keep a reference to serialize and deserialize here.
+   */
+  bson: { serialize: typeof serialize; deserialize: typeof deserialize };
 
   /**
    * @param seedlist - a list of HostAddress instances to connect to
@@ -195,6 +205,11 @@ export class Topology extends EventEmitter {
   constructor(seeds: string | string[] | HostAddress | HostAddress[], options: TopologyOptions) {
     super();
 
+    // Legacy CSFLE support
+    this.bson = Object.create(null);
+    this.bson.serialize = serialize;
+    this.bson.deserialize = deserialize;
+
     // Options should only be undefined in tests, MongoClient will always have defined options
     options = options ?? {
       hosts: [HostAddress.fromString('localhost:27017')],
diff --git a/test/unit/legacy_compat.test.js b/test/unit/legacy_compat.test.js
new file mode 100644
index 00000000000..8d7a4e0f8d9
--- /dev/null
+++ b/test/unit/legacy_compat.test.js
@@ -0,0 +1,30 @@
+'use strict';
+
+const { expect } = require('chai');
+const { MongoClient } = require('../../src');
+
+describe('Legacy 3.x features', function () {
+  it('Should have bson defined on topology', function () {
+    const client = new MongoClient(this.configuration.url());
+    return client
+      .connect()
+      .then(client => {
+        expect(client.topology).to.have.property('bson');
+        expect(client.topology.bson).to.have.property('serialize');
+        expect(client.topology.bson).to.have.property('deserialize');
+      })
+      .finally(() => client.close());
+  });
+
+  it('Should allow legacy option useUnifiedTopology', function () {
+    const url = this.configuration.url();
+    expect(() => new MongoClient(url, { useUnifiedTopology: true })).to.not.throw;
+    expect(() => new MongoClient(url, { useUnifiedTopology: false })).to.not.throw;
+  });
+
+  it('Should allow legacy option useNewUrlParser', function () {
+    const url = this.configuration.url();
+    expect(() => new MongoClient(url, { useNewUrlParser: true })).to.not.throw;
+    expect(() => new MongoClient(url, { useNewUrlParser: false })).to.not.throw;
+  });
+});