diff --git a/lib/shared/dm-ops/dm-op-specs.js b/lib/shared/dm-ops/dm-op-specs.js
index 8d9a6f78d..0f186be29 100644
--- a/lib/shared/dm-ops/dm-op-specs.js
+++ b/lib/shared/dm-ops/dm-op-specs.js
@@ -1,23 +1,25 @@
 // @flow
 
 import { addMembersSpec } from './add-members-spec.js';
 import { createSidebarSpec } from './create-sidebar-spec.js';
 import { createThreadSpec } from './create-thread-spec.js';
 import type { DMOperationSpec } from './dm-op-spec.js';
 import { joinThreadSpec } from './join-thread-spec.js';
+import { leaveThreadSpec } from './leave-thread-spec.js';
 import { sendEditMessageSpec } from './send-edit-message-spec.js';
 import { sendReactionMessageSpec } from './send-reaction-message-spec.js';
 import { sendTextMessageSpec } from './send-text-message-spec.js';
 import { type DMOperationType, dmOperationTypes } from '../../types/dm-ops.js';
 
 export const dmOpSpecs: {
   +[DMOperationType]: DMOperationSpec<any>,
 } = Object.freeze({
   [dmOperationTypes.CREATE_THREAD]: createThreadSpec,
   [dmOperationTypes.CREATE_SIDEBAR]: createSidebarSpec,
   [dmOperationTypes.SEND_TEXT_MESSAGE]: sendTextMessageSpec,
   [dmOperationTypes.SEND_REACTION_MESSAGE]: sendReactionMessageSpec,
   [dmOperationTypes.SEND_EDIT_MESSAGE]: sendEditMessageSpec,
   [dmOperationTypes.ADD_MEMBERS]: addMembersSpec,
   [dmOperationTypes.JOIN_THREAD]: joinThreadSpec,
+  [dmOperationTypes.LEAVE_THREAD]: leaveThreadSpec,
 });
diff --git a/lib/shared/dm-ops/leave-thread-spec.js b/lib/shared/dm-ops/leave-thread-spec.js
new file mode 100644
index 000000000..e7a66f4dc
--- /dev/null
+++ b/lib/shared/dm-ops/leave-thread-spec.js
@@ -0,0 +1,86 @@
+// @flow
+
+import uuid from 'uuid';
+
+import type {
+  DMOperationSpec,
+  ProcessDMOperationUtilities,
+} from './dm-op-spec.js';
+import type { DMLeaveThreadOperation } from '../../types/dm-ops.js';
+import { messageTypes } from '../../types/message-types-enum.js';
+import type { ThickRawThreadInfo } from '../../types/minimally-encoded-thread-permissions-types.js';
+import { threadTypes } from '../../types/thread-types-enum.js';
+import { updateTypes } from '../../types/update-types-enum.js';
+import type { ClientUpdateInfo } from '../../types/update-types.js';
+import { userIsMember } from '../thread-utils.js';
+
+const leaveThreadSpec: DMOperationSpec<DMLeaveThreadOperation> = Object.freeze({
+  processDMOperation: async (
+    dmOperation: DMLeaveThreadOperation,
+    viewerID: string,
+    utilities: ProcessDMOperationUtilities,
+  ) => {
+    const { editorID, time, messageID, threadID } = dmOperation;
+
+    const threadInfoOptional = utilities.threadInfos[threadID];
+    if (!threadInfoOptional || !threadInfoOptional.thick) {
+      // We can't perform this operation now. It should be queued for later.
+      return {
+        rawMessageInfos: [],
+        updateInfos: [],
+      };
+    }
+    const threadInfo: ThickRawThreadInfo = threadInfoOptional;
+
+    const leaveThreadMessage = {
+      type: messageTypes.LEAVE_THREAD,
+      id: messageID,
+      threadID,
+      creatorID: editorID,
+      time,
+    };
+
+    const updateInfos: Array<ClientUpdateInfo> = [];
+    if (
+      viewerID === editorID &&
+      userIsMember(threadInfo, editorID) &&
+      (threadInfo.type !== threadTypes.THICK_SIDEBAR ||
+        (threadInfo.parentThreadID &&
+          !utilities.threadInfos[threadInfo.parentThreadID]))
+    ) {
+      updateInfos.push({
+        type: updateTypes.DELETE_THREAD,
+        id: uuid.v4(),
+        time,
+        threadID,
+      });
+    } else {
+      const updatedThreadInfo = {
+        ...threadInfo,
+        members: threadInfo.members.filter(member => member.id !== editorID),
+      };
+      updateInfos.push(
+        {
+          type: updateTypes.UPDATE_THREAD,
+          id: uuid.v4(),
+          time,
+          threadInfo: updatedThreadInfo,
+        },
+        {
+          type: updateTypes.UPDATE_THREAD_READ_STATUS,
+          id: uuid.v4(),
+          time,
+          threadID,
+          unread: true,
+        },
+      );
+    }
+
+    return {
+      rawMessageInfos: [leaveThreadMessage],
+      updateInfos,
+    };
+  },
+});
+
+export { leaveThreadSpec };
diff --git a/lib/types/dm-ops.js b/lib/types/dm-ops.js
index faca7ce0d..ad5688b7e 100644
--- a/lib/types/dm-ops.js
+++ b/lib/types/dm-ops.js
@@ -1,243 +1,262 @@
 // @flow
 
 import t, { type TInterface, type TUnion } from 'tcomb';
 
 import { clientAvatarValidator, type ClientAvatar } from './avatar-types.js';
 import type { RawMessageInfo } from './message-types.js';
 import type { OutboundP2PMessage } from './sqlite-types.js';
 import {
   type NonSidebarThickThreadType,
   nonSidebarThickThreadTypes,
   type ThickThreadType,
   thickThreadTypeValidator,
 } from './thread-types-enum.js';
 import type { ClientUpdateInfo } from './update-types.js';
 import { values } from '../utils/objects.js';
 import { tShape, tString, tUserID } from '../utils/validation-utils.js';
 
 export const dmOperationTypes = Object.freeze({
   CREATE_THREAD: 'create_thread',
   CREATE_SIDEBAR: 'create_sidebar',
   SEND_TEXT_MESSAGE: 'send_text_message',
   SEND_REACTION_MESSAGE: 'send_reaction_message',
   SEND_EDIT_MESSAGE: 'send_edit_message',
   ADD_MEMBERS: 'add_members',
   JOIN_THREAD: 'join_thread',
+  LEAVE_THREAD: 'leave_thread',
 });
 export type DMOperationType = $Values<typeof dmOperationTypes>;
 
 export type CreateThickRawThreadInfoInput = {
   +threadID: string,
   +threadType: ThickThreadType,
   +creationTime: number,
   +parentThreadID?: ?string,
   +allMemberIDs: $ReadOnlyArray<string>,
   +roleID: string,
   +creatorID: string,
   +name?: ?string,
   +avatar?: ?ClientAvatar,
   +description?: ?string,
   +color?: ?string,
   +containingThreadID?: ?string,
   +community?: ?string,
   +sourceMessageID?: ?string,
   +repliesCount?: ?number,
   +pinnedCount?: ?number,
 };
 export const createThickRawThreadInfoInputValidator: TInterface<CreateThickRawThreadInfoInput> =
   tShape<CreateThickRawThreadInfoInput>({
     threadID: t.String,
     threadType: thickThreadTypeValidator,
     creationTime: t.Number,
     parentThreadID: t.maybe(t.String),
     allMemberIDs: t.list(tUserID),
     roleID: t.String,
     creatorID: tUserID,
     name: t.maybe(t.String),
     avatar: t.maybe(clientAvatarValidator),
     description: t.maybe(t.String),
     color: t.maybe(t.String),
     containingThreadID: t.maybe(t.String),
     community: t.maybe(t.String),
     sourceMessageID: t.maybe(t.String),
     repliesCount: t.maybe(t.Number),
     pinnedCount: t.maybe(t.Number),
   });
 
 export type DMCreateThreadOperation = {
   +type: 'create_thread',
   +threadID: string,
   +creatorID: string,
   +time: number,
   +threadType: NonSidebarThickThreadType,
   +memberIDs: $ReadOnlyArray<string>,
   +roleID: string,
   +newMessageID: string,
 };
 export const dmCreateThreadOperationValidator: TInterface<DMCreateThreadOperation> =
   tShape<DMCreateThreadOperation>({
     type: tString(dmOperationTypes.CREATE_THREAD),
     threadID: t.String,
     creatorID: tUserID,
     time: t.Number,
     threadType: t.enums.of(values(nonSidebarThickThreadTypes)),
     memberIDs: t.list(tUserID),
     roleID: t.String,
     newMessageID: t.String,
   });
 
 export type DMCreateSidebarOperation = {
   +type: 'create_sidebar',
   +threadID: string,
   +creatorID: string,
   +time: number,
   +parentThreadID: string,
   +memberIDs: $ReadOnlyArray<string>,
   +sourceMessageID: string,
   +roleID: string,
   +newSidebarSourceMessageID: string,
   +newCreateSidebarMessageID: string,
 };
 export const dmCreateSidebarOperationValidator: TInterface<DMCreateSidebarOperation> =
   tShape<DMCreateSidebarOperation>({
     type: tString(dmOperationTypes.CREATE_SIDEBAR),
     threadID: t.String,
     creatorID: tUserID,
     time: t.Number,
     parentThreadID: t.String,
     memberIDs: t.list(tUserID),
     sourceMessageID: t.String,
     roleID: t.String,
     newSidebarSourceMessageID: t.String,
     newCreateSidebarMessageID: t.String,
   });
 
 export type DMSendTextMessageOperation = {
   +type: 'send_text_message',
   +threadID: string,
   +creatorID: string,
   +time: number,
   +messageID: string,
   +text: string,
 };
 export const dmSendTextMessageOperationValidator: TInterface<DMSendTextMessageOperation> =
   tShape<DMSendTextMessageOperation>({
     type: tString(dmOperationTypes.SEND_TEXT_MESSAGE),
     threadID: t.String,
     creatorID: tUserID,
     time: t.Number,
     messageID: t.String,
     text: t.String,
   });
 
 export type DMSendReactionMessageOperation = {
   +type: 'send_reaction_message',
   +threadID: string,
   +creatorID: string,
   +time: number,
   +messageID: string,
   +targetMessageID: string,
   +reaction: string,
   +action: 'add_reaction' | 'remove_reaction',
 };
 export const dmSendReactionMessageOperationValidator: TInterface<DMSendReactionMessageOperation> =
   tShape<DMSendReactionMessageOperation>({
     type: tString(dmOperationTypes.SEND_REACTION_MESSAGE),
     threadID: t.String,
     creatorID: tUserID,
     time: t.Number,
     messageID: t.String,
     targetMessageID: t.String,
     reaction: t.String,
     action: t.enums.of(['add_reaction', 'remove_reaction']),
   });
 
 export type DMSendEditMessageOperation = {
   +type: 'send_edit_message',
   +threadID: string,
   +creatorID: string,
   +time: number,
   +messageID: string,
   +targetMessageID: string,
   +text: string,
 };
 export const dmSendEditMessageOperationValidator: TInterface<DMSendEditMessageOperation> =
   tShape<DMSendEditMessageOperation>({
     type: tString(dmOperationTypes.SEND_EDIT_MESSAGE),
     threadID: t.String,
     creatorID: tUserID,
     time: t.Number,
     messageID: t.String,
     targetMessageID: t.String,
     text: t.String,
   });
 
 export type DMAddMembersOperation = {
   +type: 'add_members',
   +editorID: string,
   +time: number,
   +messageID: string,
   +addedUserIDs: $ReadOnlyArray<string>,
   +existingThreadDetails: CreateThickRawThreadInfoInput,
 };
 export const dmAddMembersOperationValidator: TInterface<DMAddMembersOperation> =
   tShape<DMAddMembersOperation>({
     type: tString(dmOperationTypes.ADD_MEMBERS),
     editorID: tUserID,
     time: t.Number,
     messageID: t.String,
     addedUserIDs: t.list(tUserID),
     existingThreadDetails: createThickRawThreadInfoInputValidator,
   });
 
 export type DMJoinThreadOperation = {
   +type: 'join_thread',
   +editorID: string,
   +time: number,
   +messageID: string,
   +existingThreadDetails: CreateThickRawThreadInfoInput,
 };
 export const dmJoinThreadOperationValidator: TInterface<DMJoinThreadOperation> =
   tShape<DMJoinThreadOperation>({
     type: tString(dmOperationTypes.JOIN_THREAD),
     editorID: tUserID,
     time: t.Number,
     messageID: t.String,
     existingThreadDetails: createThickRawThreadInfoInputValidator,
   });
 
+export type DMLeaveThreadOperation = {
+  +type: 'leave_thread',
+  +editorID: string,
+  +time: number,
+  +messageID: string,
+  +threadID: string,
+};
+export const dmLeaveThreadOperationValidator: TInterface<DMLeaveThreadOperation> =
+  tShape<DMLeaveThreadOperation>({
+    type: tString(dmOperationTypes.LEAVE_THREAD),
+    editorID: tUserID,
+    time: t.Number,
+    messageID: t.String,
+    threadID: t.String,
+  });
+
 export type DMOperation =
   | DMCreateThreadOperation
   | DMCreateSidebarOperation
   | DMSendTextMessageOperation
   | DMSendReactionMessageOperation
   | DMSendEditMessageOperation
   | DMAddMembersOperation
-  | DMJoinThreadOperation;
+  | DMJoinThreadOperation
+  | DMLeaveThreadOperation;
 export const dmOperationValidator: TUnion<DMOperation> = t.union([
   dmCreateThreadOperationValidator,
   dmCreateSidebarOperationValidator,
   dmSendTextMessageOperationValidator,
   dmSendReactionMessageOperationValidator,
   dmSendEditMessageOperationValidator,
   dmAddMembersOperationValidator,
   dmJoinThreadOperationValidator,
+  dmLeaveThreadOperationValidator,
 ]);
 
 export type DMOperationResult = {
   rawMessageInfos: Array<RawMessageInfo>,
   updateInfos: Array<ClientUpdateInfo>,
 };
 
 export const processDMOpsActionType = 'PROCESS_DM_OPS';
 
 export type ProcessDMOpsPayload = {
   +rawMessageInfos: $ReadOnlyArray<RawMessageInfo>,
   +updateInfos: $ReadOnlyArray<ClientUpdateInfo>,
 };
 
 export const scheduleP2PMessagesActionType = 'SCHEDULE_P2P_MESSAGES';
 export type ScheduleP2PMessagesPayload = {
   +dmOpID: string,
   +messages: $ReadOnlyArray<OutboundP2PMessage>,
 };