diff --git a/spec/unit/matrix-client.spec.ts b/spec/unit/matrix-client.spec.ts
index 68c6ded9f..58e6a5799 100644
--- a/spec/unit/matrix-client.spec.ts
+++ b/spec/unit/matrix-client.spec.ts
@@ -22,11 +22,13 @@ import { Filter } from "../../src/filter";
 import { DEFAULT_TREE_POWER_LEVELS_TEMPLATE } from "../../src/models/MSC3089TreeSpace";
 import {
     EventType,
+    RelationType,
     RoomCreateTypeField,
     RoomType,
     UNSTABLE_MSC3088_ENABLED,
     UNSTABLE_MSC3088_PURPOSE,
     UNSTABLE_MSC3089_TREE_SUBTYPE,
+    MSC3912_RELATION_BASED_REDACTIONS_PROP,
 } from "../../src/@types/event";
 import { MEGOLM_ALGORITHM } from "../../src/crypto/olmlib";
 import { Crypto } from "../../src/crypto";
@@ -121,6 +123,10 @@ describe("MatrixClient", function () {
         data: SYNC_DATA,
     };
 
+    const unstableFeatures: Record<string, boolean> = {
+        "org.matrix.msc3440.stable": true,
+    };
+
     // items are popped off when processed and block if no items left.
     let httpLookups: HttpLookup[] = [];
     let acceptKeepalives: boolean;
@@ -132,9 +138,7 @@ describe("MatrixClient", function () {
     function httpReq(method: Method, path: string, qp?: QueryDict, data?: BodyInit, opts?: IRequestOpts) {
         if (path === KEEP_ALIVE_PATH && acceptKeepalives) {
             return Promise.resolve({
-                unstable_features: {
-                    "org.matrix.msc3440.stable": true,
-                },
+                unstable_features: unstableFeatures,
                 versions: ["r0.6.0", "r0.6.1"],
             });
         }
@@ -1085,6 +1089,59 @@ describe("MatrixClient", function () {
 
             await client.redactEvent(roomId, eventId, txnId, { reason });
         });
+
+        describe("when calling with with_relations", () => {
+            const eventId = "$event42:example.org";
+
+            it("should raise an error if server has no support for relation based redactions", async () => {
+                // load supported features
+                await client.getVersions();
+
+                const txnId = client.makeTxnId();
+
+                expect(() => {
+                    client.redactEvent(roomId, eventId, txnId, {
+                        with_relations: [RelationType.Reference],
+                    });
+                }).toThrowError(
+                    new Error(
+                        "Server does not support relation based redactions " +
+                            `roomId ${roomId} eventId ${eventId} txnId: ${txnId} threadId null`,
+                    ),
+                );
+            });
+
+            describe("and the server supports relation based redactions (unstable)", () => {
+                beforeEach(async () => {
+                    unstableFeatures["org.matrix.msc3912"] = true;
+                    // load supported features
+                    await client.getVersions();
+                });
+
+                it("should send with_relations in the request body", async () => {
+                    const txnId = client.makeTxnId();
+
+                    httpLookups = [
+                        {
+                            method: "PUT",
+                            path:
+                                `/rooms/${encodeURIComponent(roomId)}/redact/${encodeURIComponent(eventId)}` +
+                                `/${encodeURIComponent(txnId)}`,
+                            expectBody: {
+                                reason: "redaction test",
+                                [MSC3912_RELATION_BASED_REDACTIONS_PROP.unstable!]: [RelationType.Reference],
+                            },
+                            data: { event_id: eventId },
+                        },
+                    ];
+
+                    await client.redactEvent(roomId, eventId, txnId, {
+                        reason: "redaction test",
+                        with_relations: [RelationType.Reference],
+                    });
+                });
+            });
+        });
     });
 
     describe("cancelPendingEvent", () => {
diff --git a/src/@types/event.ts b/src/@types/event.ts
index 230cb0503..4d7cd5d51 100644
--- a/src/@types/event.ts
+++ b/src/@types/event.ts
@@ -165,6 +165,15 @@ export const UNSTABLE_MSC3089_BRANCH = new UnstableValue("m.branch", "org.matrix
  */
 export const UNSTABLE_MSC2716_MARKER = new UnstableValue("m.room.marker", "org.matrix.msc2716.marker");
 
+/**
+ * Name of the "with_relations" request property for relation based redactions.
+ * {@link https://github.com/matrix-org/matrix-spec-proposals/pull/3912}
+ */
+export const MSC3912_RELATION_BASED_REDACTIONS_PROP = new UnstableValue(
+    "with_relations",
+    "org.matrix.msc3912.with_relations",
+);
+
 /**
  * Functional members type for declaring a purpose of room members (e.g. helpful bots).
  * Note that this reference is UNSTABLE and subject to breaking changes, including its
diff --git a/src/@types/requests.ts b/src/@types/requests.ts
index 75296940a..12f4d8eb0 100644
--- a/src/@types/requests.ts
+++ b/src/@types/requests.ts
@@ -21,7 +21,7 @@ import { IRoomEventFilter } from "../filter";
 import { Direction } from "../models/event-timeline";
 import { PushRuleAction } from "./PushRules";
 import { IRoomEvent } from "../sync-accumulator";
-import { EventType, RoomType } from "./event";
+import { EventType, RelationType, RoomType } from "./event";
 
 // allow camelcase as these are things that go onto the wire
 /* eslint-disable camelcase */
@@ -47,6 +47,18 @@ export interface IJoinRoomOpts {
 
 export interface IRedactOpts {
     reason?: string;
+    /**
+     * Whether events related to the redacted event should be redacted.
+     *
+     * If specified, then any events which relate to the event being redacted with
+     * any of the relationship types listed will also be redacted.
+     *
+     * <b>Raises an Error if the server does not support it.</b>
+     * Check for server-side support before using this param with
+     * <code>client.canSupport.get(Feature.RelationBasedRedactions)</code>.
+     * {@link https://github.com/matrix-org/matrix-spec-proposals/pull/3912}
+     */
+    with_relations?: Array<RelationType | string>;
 }
 
 export interface ISendEventResponse {
diff --git a/src/client.ts b/src/client.ts
index 8066b22c8..26d31ef0c 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -154,6 +154,7 @@ import {
     UNSTABLE_MSC3088_ENABLED,
     UNSTABLE_MSC3088_PURPOSE,
     UNSTABLE_MSC3089_TREE_SUBTYPE,
+    MSC3912_RELATION_BASED_REDACTIONS_PROP,
 } from "./@types/event";
 import { IdServerUnbindResult, IImageInfo, Preset, Visibility } from "./@types/partials";
 import { EventMapper, eventMapperFor, MapperOpts } from "./event-mapper";
@@ -4444,9 +4445,11 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
 
     /**
      * @param txnId -  transaction id. One will be made up if not supplied.
-     * @param opts - Options to pass on, may contain `reason`.
+     * @param opts - Options to pass on, may contain `reason` and `with_relations` (MSC3912)
      * @returns Promise which resolves: TODO
      * @returns Rejects: with an error response.
+     * @throws Error if called with `with_relations` (MSC3912) but the server does not support it.
+     *         Callers should check whether the server supports MSC3912 via `MatrixClient.canSupport`.
      */
     public redactEvent(
         roomId: string,
@@ -4475,12 +4478,34 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
             threadId = null;
         }
         const reason = opts?.reason;
+
+        if (
+            opts?.with_relations &&
+            this.canSupport.get(Feature.RelationBasedRedactions) === ServerSupport.Unsupported
+        ) {
+            throw new Error(
+                "Server does not support relation based redactions " +
+                    `roomId ${roomId} eventId ${eventId} txnId: ${txnId} threadId ${threadId}`,
+            );
+        }
+
+        const withRelations = opts?.with_relations
+            ? {
+                  [this.canSupport.get(Feature.RelationBasedRedactions) === ServerSupport.Stable
+                      ? MSC3912_RELATION_BASED_REDACTIONS_PROP.stable!
+                      : MSC3912_RELATION_BASED_REDACTIONS_PROP.unstable!]: opts?.with_relations,
+              }
+            : {};
+
         return this.sendCompleteEvent(
             roomId,
             threadId,
             {
                 type: EventType.RoomRedaction,
-                content: { reason },
+                content: {
+                    ...withRelations,
+                    reason,
+                },
                 redacts: eventId,
             },
             txnId as string,
diff --git a/src/feature.ts b/src/feature.ts
index 158e1f7ee..9141e811c 100644
--- a/src/feature.ts
+++ b/src/feature.ts
@@ -26,6 +26,7 @@ export enum Feature {
     Thread = "Thread",
     ThreadUnreadNotifications = "ThreadUnreadNotifications",
     LoginTokenRequest = "LoginTokenRequest",
+    RelationBasedRedactions = "RelationBasedRedactions",
     AccountDataDeletion = "AccountDataDeletion",
 }
 
@@ -46,6 +47,9 @@ const featureSupportResolver: Record<string, FeatureSupportCondition> = {
     [Feature.LoginTokenRequest]: {
         unstablePrefixes: ["org.matrix.msc3882"],
     },
+    [Feature.RelationBasedRedactions]: {
+        unstablePrefixes: ["org.matrix.msc3912"],
+    },
     [Feature.AccountDataDeletion]: {
         unstablePrefixes: ["org.matrix.msc3391"],
     },