diff --git a/spec/unit/rust-crypto/RoomEncryptor.spec.ts b/spec/unit/rust-crypto/RoomEncryptor.spec.ts
index 66d21f9d5da..de6863cbce1 100644
--- a/spec/unit/rust-crypto/RoomEncryptor.spec.ts
+++ b/spec/unit/rust-crypto/RoomEncryptor.spec.ts
@@ -16,16 +16,125 @@
  * /
  */
 
-import { HistoryVisibility as RustHistoryVisibility } from "@matrix-org/matrix-sdk-crypto-wasm";
-
-import { HistoryVisibility } from "../../../src";
-import { toRustHistoryVisibility } from "../../../src/rust-crypto/RoomEncryptor";
-
-it.each([
-    [HistoryVisibility.Invited, RustHistoryVisibility.Invited],
-    [HistoryVisibility.Joined, RustHistoryVisibility.Joined],
-    [HistoryVisibility.Shared, RustHistoryVisibility.Shared],
-    [HistoryVisibility.WorldReadable, RustHistoryVisibility.WorldReadable],
-])("JS HistoryVisibility to Rust HistoryVisibility: converts %s to %s", (historyVisibility, expected) => {
-    expect(toRustHistoryVisibility(historyVisibility)).toBe(expected);
+import {
+    Curve25519PublicKey,
+    Ed25519PublicKey,
+    HistoryVisibility as RustHistoryVisibility,
+    IdentityKeys,
+    OlmMachine,
+} from "@matrix-org/matrix-sdk-crypto-wasm";
+import { Mocked } from "jest-mock";
+
+import { HistoryVisibility, MatrixEvent, Room, RoomMember } from "../../../src";
+import { RoomEncryptor, toRustHistoryVisibility } from "../../../src/rust-crypto/RoomEncryptor";
+import { KeyClaimManager } from "../../../src/rust-crypto/KeyClaimManager";
+import { defer } from "../../../src/utils";
+import { OutgoingRequestsManager } from "../../../src/rust-crypto/OutgoingRequestsManager";
+
+describe("RoomEncryptor", () => {
+    describe("History Visibility", () => {
+        it.each([
+            [HistoryVisibility.Invited, RustHistoryVisibility.Invited],
+            [HistoryVisibility.Joined, RustHistoryVisibility.Joined],
+            [HistoryVisibility.Shared, RustHistoryVisibility.Shared],
+            [HistoryVisibility.WorldReadable, RustHistoryVisibility.WorldReadable],
+        ])("JS HistoryVisibility to Rust HistoryVisibility: converts %s to %s", (historyVisibility, expected) => {
+            expect(toRustHistoryVisibility(historyVisibility)).toBe(expected);
+        });
+    });
+
+    describe("RoomEncryptor", () => {
+        /** The room encryptor under test */
+        let roomEncryptor: RoomEncryptor;
+
+        let mockOlmMachine: Mocked<OlmMachine>;
+        let mockKeyClaimManager: Mocked<KeyClaimManager>;
+        let mockOutgoingRequestManager: Mocked<OutgoingRequestsManager>;
+        let mockRoom: Mocked<Room>;
+
+        function createMockEvent(text: string): Mocked<MatrixEvent> {
+            return {
+                getTxnId: jest.fn().mockReturnValue(""),
+                getType: jest.fn().mockReturnValue("m.room.message"),
+                getContent: jest.fn().mockReturnValue({
+                    body: text,
+                    msgtype: "m.text",
+                }),
+                makeEncrypted: jest.fn().mockReturnValue(undefined),
+            } as unknown as Mocked<MatrixEvent>;
+        }
+
+        beforeEach(() => {
+            mockOlmMachine = {
+                identityKeys: {
+                    curve25519: {
+                        toBase64: jest.fn().mockReturnValue("curve25519"),
+                    } as unknown as Curve25519PublicKey,
+                    ed25519: {
+                        toBase64: jest.fn().mockReturnValue("ed25519"),
+                    } as unknown as Ed25519PublicKey,
+                } as unknown as Mocked<IdentityKeys>,
+                shareRoomKey: jest.fn(),
+                updateTrackedUsers: jest.fn().mockResolvedValue(undefined),
+                encryptRoomEvent: jest.fn().mockResolvedValue("{}"),
+            } as unknown as Mocked<OlmMachine>;
+
+            mockKeyClaimManager = {
+                ensureSessionsForUsers: jest.fn(),
+            } as unknown as Mocked<KeyClaimManager>;
+
+            mockOutgoingRequestManager = {
+                doProcessOutgoingRequests: jest.fn().mockResolvedValue(undefined),
+            } as unknown as Mocked<OutgoingRequestsManager>;
+
+            const mockRoomMember = {
+                userId: "@alice:example.org",
+                membership: "join",
+            } as unknown as Mocked<RoomMember>;
+
+            mockRoom = {
+                roomId: "!foo:example.org",
+                getJoinedMembers: jest.fn().mockReturnValue([mockRoomMember]),
+                getEncryptionTargetMembers: jest.fn().mockReturnValue([mockRoomMember]),
+                shouldEncryptForInvitedMembers: jest.fn().mockReturnValue(true),
+                getHistoryVisibility: jest.fn().mockReturnValue(HistoryVisibility.Invited),
+                getBlacklistUnverifiedDevices: jest.fn().mockReturnValue(false),
+            } as unknown as Mocked<Room>;
+
+            roomEncryptor = new RoomEncryptor(
+                mockOlmMachine,
+                mockKeyClaimManager,
+                mockOutgoingRequestManager,
+                mockRoom,
+                { algorithm: "m.megolm.v1.aes-sha2" },
+            );
+        });
+
+        it("should ensure that there is only one shareRoomKey at a time", async () => {
+            const deferredShare = defer<void>();
+            const insideOlmShareRoom = defer<void>();
+
+            mockOlmMachine.shareRoomKey.mockImplementationOnce(async () => {
+                insideOlmShareRoom.resolve();
+                await deferredShare.promise;
+            });
+
+            roomEncryptor.prepareForEncryption(false);
+            await insideOlmShareRoom.promise;
+
+            // call several times more
+            roomEncryptor.prepareForEncryption(false);
+            roomEncryptor.encryptEvent(createMockEvent("Hello"), false);
+            roomEncryptor.prepareForEncryption(false);
+            roomEncryptor.encryptEvent(createMockEvent("World"), false);
+
+            expect(mockOlmMachine.shareRoomKey).toHaveBeenCalledTimes(1);
+
+            deferredShare.resolve();
+            await roomEncryptor.prepareForEncryption(false);
+
+            // should have been called again
+            expect(mockOlmMachine.shareRoomKey).toHaveBeenCalledTimes(6);
+        });
+    });
 });
diff --git a/src/rust-crypto/RoomEncryptor.ts b/src/rust-crypto/RoomEncryptor.ts
index 9f11f79c13b..5a752c53fdc 100644
--- a/src/rust-crypto/RoomEncryptor.ts
+++ b/src/rust-crypto/RoomEncryptor.ts
@@ -45,6 +45,9 @@ export class RoomEncryptor {
     /** whether the room members have been loaded and tracked for the first time */
     private lazyLoadedMembersResolved = false;
 
+    /** Ensures that there is only one call to shareRoomKeys at a time */
+    private currentShareRoomKeyPromise = Promise.resolve();
+
     /**
      * @param olmMachine - The rust-sdk's OlmMachine
      * @param keyClaimManager - Our KeyClaimManager, which manages the queue of one-time-key claim requests
@@ -198,11 +201,7 @@ export class RoomEncryptor {
         rustEncryptionSettings.onlyAllowTrustedDevices =
             this.room.getBlacklistUnverifiedDevices() ?? globalBlacklistUnverifiedDevices;
 
-        const shareMessages: ToDeviceRequest[] = await this.olmMachine.shareRoomKey(
-            new RoomId(this.room.roomId),
-            userList,
-            rustEncryptionSettings,
-        );
+        const shareMessages: ToDeviceRequest[] = await this.shareRoomKey(userList, rustEncryptionSettings);
         if (shareMessages) {
             for (const m of shareMessages) {
                 await this.outgoingRequestManager.outgoingRequestProcessor.makeOutgoingRequest(m);
@@ -210,6 +209,30 @@ export class RoomEncryptor {
         }
     }
 
+    /**
+     *  The Rust-SDK requires that we only have one shareRoomKey process in flight at once for a room.
+     *  This method ensures that, by only having one call to shareRoomKey active at once (and making them
+     *  queue up in order).
+     *
+     * @param userList - list of userIDs to share with
+     * @param rustEncryptionSettings - encryption settings to use
+     *
+     * @returns a promise which resolves to the list of ToDeviceRequests to send
+     */
+    private async shareRoomKey(
+        userList: UserId[],
+        rustEncryptionSettings: EncryptionSettings,
+    ): Promise<ToDeviceRequest[]> {
+        const prom = this.currentShareRoomKeyPromise
+            .catch(() => {
+                // any errors in the previous claim will have been reported already, so there is nothing to do here.
+                // we just throw away the error and start anew.
+            })
+            .then(() => this.olmMachine.shareRoomKey(new RoomId(this.room.roomId), userList, rustEncryptionSettings));
+        this.currentShareRoomKeyPromise = prom;
+        return prom;
+    }
+
     /**
      * Discard any existing group session for this room
      */