diff --git a/modules/signals/entities/src/helpers.ts b/modules/signals/entities/src/helpers.ts
index 0ed58ccb4a..d14dc8b69c 100644
--- a/modules/signals/entities/src/helpers.ts
+++ b/modules/signals/entities/src/helpers.ts
@@ -2,6 +2,7 @@ import {
   DidMutate,
   EntityChanges,
   EntityId,
+  EntityMap,
   EntityPredicate,
   EntityState,
   SelectEntityId,
@@ -10,6 +11,11 @@ import {
 declare const ngDevMode: unknown;
 const defaultSelectId: SelectEntityId<{ id: EntityId }> = (entity) => entity.id;
 
+type EntityStateMutable<Entity = any> = {
+  entityMap: Record<EntityId, Entity>;
+  ids: EntityId[];
+};
+
 export function getEntityIdSelector(config?: {
   selectId?: SelectEntityId<any>;
 }): SelectEntityId<any> {
@@ -79,8 +85,8 @@ export function addEntityMutably(
     return DidMutate.None;
   }
 
-  state.entityMap[id] = entity;
-  state.ids.push(id);
+  (state as EntityStateMutable).entityMap[id] = entity;
+  (state as EntityStateMutable).ids.push(id);
 
   return DidMutate.Both;
 }
@@ -111,12 +117,12 @@ export function setEntityMutably(
   const id = selectId(entity);
 
   if (state.entityMap[id]) {
-    state.entityMap[id] = entity;
+    (state as EntityStateMutable).entityMap[id] = entity;
     return DidMutate.Entities;
   }
 
-  state.entityMap[id] = entity;
-  state.ids.push(id);
+  (state as EntityStateMutable).entityMap[id] = entity;
+  (state as EntityStateMutable).ids.push(id);
 
   return DidMutate.Both;
 }
@@ -152,13 +158,15 @@ export function removeEntitiesMutably(
 
   for (const id of ids) {
     if (state.entityMap[id]) {
-      delete state.entityMap[id];
+      delete (state as EntityStateMutable).entityMap[id];
       didMutate = DidMutate.Both;
     }
   }
 
   if (didMutate === DidMutate.Both) {
-    state.ids = state.ids.filter((id) => id in state.entityMap);
+    (state as EntityStateMutable).ids = state.ids.filter(
+      (id) => id in state.entityMap
+    );
   }
 
   return didMutate;
@@ -182,13 +190,16 @@ export function updateEntitiesMutably(
     if (entity) {
       const changesRecord =
         typeof changes === 'function' ? changes(entity) : changes;
-      state.entityMap[id] = { ...entity, ...changesRecord };
+      (state as EntityStateMutable).entityMap[id] = {
+        ...entity,
+        ...changesRecord,
+      };
       didMutate = DidMutate.Entities;
 
       const newId = selectId(state.entityMap[id]);
       if (newId !== id) {
-        state.entityMap[newId] = state.entityMap[id];
-        delete state.entityMap[id];
+        (state as EntityStateMutable).entityMap[newId] = state.entityMap[id];
+        delete (state as EntityStateMutable).entityMap[id];
 
         newIds = newIds || {};
         newIds[id] = newId;
@@ -197,7 +208,7 @@ export function updateEntitiesMutably(
   }
 
   if (newIds) {
-    state.ids = state.ids.map((id) => newIds[id] ?? id);
+    (state as EntityStateMutable).ids = state.ids.map((id) => newIds[id] ?? id);
     didMutate = DidMutate.Both;
   }
 
diff --git a/modules/signals/entities/src/models.ts b/modules/signals/entities/src/models.ts
index 956311b50b..2fd58d7625 100644
--- a/modules/signals/entities/src/models.ts
+++ b/modules/signals/entities/src/models.ts
@@ -2,11 +2,11 @@ import { Signal } from '@angular/core';
 
 export type EntityId = string | number;
 
-export type EntityMap<Entity> = Record<EntityId, Entity>;
+export type EntityMap<Entity> = Readonly<Record<EntityId, Entity>>;
 
 export type EntityState<Entity> = {
-  entityMap: EntityMap<Entity>;
-  ids: EntityId[];
+  readonly entityMap: EntityMap<Entity>;
+  readonly ids: readonly EntityId[];
 };
 
 export type NamedEntityState<Entity, Collection extends string> = {
@@ -14,7 +14,7 @@ export type NamedEntityState<Entity, Collection extends string> = {
 };
 
 export type EntityProps<Entity> = {
-  entities: Signal<Entity[]>;
+  entities: Signal<readonly Entity[]>;
 };
 
 export type NamedEntityProps<Entity, Collection extends string> = {
diff --git a/modules/signals/spec/types/signal-store.types.spec.ts b/modules/signals/spec/types/signal-store.types.spec.ts
index 6854f27064..1d6594e6b4 100644
--- a/modules/signals/spec/types/signal-store.types.spec.ts
+++ b/modules/signals/spec/types/signal-store.types.spec.ts
@@ -34,7 +34,7 @@ describe('signalStore', () => {
 
     expectSnippet(snippet).toInfer(
       'Store',
-      'Type<{ foo: Signal<string>; bar: Signal<number[]>; } & StateSource<{ foo: string; bar: number[]; }>>'
+      'Type<{ readonly foo: Signal<string>; readonly bar: Signal<number[]>; } & StateSource<{ foo: string; bar: number[]; }>>'
     );
   });
 
@@ -64,7 +64,7 @@ describe('signalStore', () => {
 
     expectSnippet(snippet).toInfer(
       'store',
-      '{ user: DeepSignal<{ age: number; details: { first: string; flags: boolean[]; }; }>; } & StateSource<{ user: { age: number; details: { first: string; flags: boolean[]; }; }; }>'
+      '{ readonly user: DeepSignal<{ age: number; details: { first: string; flags: boolean[]; }; }>; } & StateSource<{ user: { age: number; details: { first: string; flags: boolean[]; }; }; }>'
     );
 
     expectSnippet(snippet).toInfer(
@@ -265,7 +265,7 @@ describe('signalStore', () => {
 
     expectSnippet(snippet).toInfer(
       'store',
-      '{ foo: Signal<number | { s: string; }>; bar: DeepSignal<{ baz: { b: boolean; } | null; }>; x: DeepSignal<{ y: { z: number | undefined; }; }>; } & StateSource<{ foo: number | { ...; }; bar: { ...; }; x: { ...; }; }>'
+      '{ readonly foo: Signal<number | { s: string; }>; readonly bar: DeepSignal<{ baz: { b: boolean; } | null; }>; readonly x: DeepSignal<{ y: { z: number | undefined; }; }>; } & StateSource<...>'
     );
 
     expectSnippet(snippet).toInfer('foo', 'Signal<number | { s: string; }>');
@@ -305,7 +305,7 @@ describe('signalStore', () => {
 
     expectSnippet(snippet1).toInfer(
       'Store',
-      'Type<{ name: DeepSignal<{ x: { y: string; }; }>; arguments: Signal<number[]>; call: Signal<boolean>; } & StateSource<{ name: { x: { y: string; }; }; arguments: number[]; call: boolean; }>>'
+      'Type<{ readonly name: DeepSignal<{ x: { y: string; }; }>; readonly arguments: Signal<number[]>; readonly call: Signal<boolean>; } & StateSource<{ name: { x: { y: string; }; }; arguments: number[]; call: boolean; }>>'
     );
 
     const snippet2 = `
@@ -322,7 +322,7 @@ describe('signalStore', () => {
 
     expectSnippet(snippet2).toInfer(
       'Store',
-      'Type<{ apply: Signal<string>; bind: DeepSignal<{ foo: string; }>; prototype: Signal<string[]>; } & StateSource<{ apply: string; bind: { foo: string; }; prototype: string[]; }>>'
+      'Type<{ readonly apply: Signal<string>; readonly bind: DeepSignal<{ foo: string; }>; readonly prototype: Signal<string[]>; } & StateSource<{ apply: string; bind: { foo: string; }; prototype: string[]; }>>'
     );
 
     const snippet3 = `
@@ -338,7 +338,7 @@ describe('signalStore', () => {
 
     expectSnippet(snippet3).toInfer(
       'Store',
-      'Type<{ length: Signal<number>; caller: Signal<undefined>; } & StateSource<{ length: number; caller: undefined; }>>'
+      'Type<{ readonly length: Signal<number>; readonly caller: Signal<undefined>; } & StateSource<{ length: number; caller: undefined; }>>'
     );
   });
 
@@ -393,7 +393,7 @@ describe('signalStore', () => {
 
     expectSnippet(snippet).toInfer(
       'store',
-      '{ bar: DeepSignal<{ baz?: number | undefined; }>; x: DeepSignal<{ y?: { z: boolean; } | undefined; }>; } & StateSource<{ bar: { baz?: number | undefined; }; x: { y?: { z: boolean; } | undefined; }; }>'
+      '{ readonly bar: DeepSignal<{ baz?: number | undefined; }>; readonly x: DeepSignal<{ y?: { z: boolean; } | undefined; }>; } & StateSource<{ bar: { baz?: number | undefined; }; x: { ...; }; }>'
     );
 
     expectSnippet(snippet).toInfer(
@@ -503,7 +503,7 @@ describe('signalStore', () => {
 
     expectSnippet(snippet).toInfer(
       'store1',
-      '{ count: Signal<number>; } & StateSource<{ count: number; }>'
+      '{ readonly count: Signal<number>; } & StateSource<{ count: number; }>'
     );
 
     expectSnippet(snippet).toInfer('state1', '{ count: number; }');
@@ -515,7 +515,7 @@ describe('signalStore', () => {
 
     expectSnippet(snippet).toInfer(
       'store2',
-      '{ count: Signal<number>; } & StateSource<{ count: number; }>'
+      '{ readonly count: Signal<number>; } & StateSource<{ count: number; }>'
     );
 
     expectSnippet(snippet).toInfer('state2', '{ count: number; }');
@@ -548,7 +548,7 @@ describe('signalStore', () => {
 
     expectSnippet(snippet).toInfer(
       'store1',
-      '{ count: Signal<number>; } & StateSource<{ count: number; }>'
+      '{ readonly count: Signal<number>; } & StateSource<{ count: number; }>'
     );
 
     expectSnippet(snippet).toInfer('state1', '{ count: number; }');
@@ -560,7 +560,7 @@ describe('signalStore', () => {
 
     expectSnippet(snippet).toInfer(
       'store2',
-      '{ count: Signal<number>; } & StateSource<{ count: number; }>'
+      '{ readonly count: Signal<number>; } & StateSource<{ count: number; }>'
     );
 
     expectSnippet(snippet).toInfer('state2', '{ count: number; }');
@@ -593,14 +593,14 @@ describe('signalStore', () => {
 
     expectSnippet(snippet).toInfer(
       'store1',
-      '{ count: Signal<number>; } & WritableStateSource<{ count: number; }>'
+      '{ readonly count: Signal<number>; } & WritableStateSource<{ count: number; }>'
     );
 
     expectSnippet(snippet).toInfer('state1', '{ count: number; }');
 
     expectSnippet(snippet).toInfer(
       'store2',
-      '{ count: Signal<number>; } & WritableStateSource<{ count: number; }>'
+      '{ readonly count: Signal<number>; } & WritableStateSource<{ count: number; }>'
     );
 
     expectSnippet(snippet).toInfer('state2', '{ count: number; }');
@@ -762,7 +762,7 @@ describe('signalStore', () => {
 
     expectSnippet(snippet).toInfer(
       'store',
-      '{ ngrx: Signal<string>; x: DeepSignal<{ y: string; }>; signals: Signal<number[]>; mgmt: (arg: boolean) => number; } & StateSource<{ ngrx: string; x: { y: string; }; }>'
+      '{ readonly ngrx: Signal<string>; readonly x: DeepSignal<{ y: string; }>; readonly signals: Signal<number[]>; readonly mgmt: (arg: boolean) => number; } & StateSource<...>'
     );
   });
 
@@ -790,7 +790,7 @@ describe('signalStore', () => {
 
     expectSnippet(snippet).toInfer(
       'store',
-      '{ foo: Signal<number>; bar: Signal<string>; baz: (x: number) => void; } & StateSource<{ foo: number; }>'
+      '{ readonly foo: Signal<number>; readonly bar: Signal<string>; readonly baz: (x: number) => void; } & StateSource<{ foo: number; }>'
     );
   });
 
@@ -838,7 +838,7 @@ describe('signalStore', () => {
 
     expectSnippet(snippet).toInfer(
       'store',
-      '{ count1: Signal<number>; doubleCount2: Signal<number>; increment1: () => void; } & StateSource<{ count1: number; }>'
+      '{ readonly count1: Signal<number>; readonly doubleCount2: Signal<number>; readonly increment1: () => void; } & StateSource<{ count1: number; }>'
     );
   });
 
diff --git a/modules/signals/src/signal-store.ts b/modules/signals/src/signal-store.ts
index ce70ab8e46..aef83b7759 100644
--- a/modules/signals/src/signal-store.ts
+++ b/modules/signals/src/signal-store.ts
@@ -13,10 +13,12 @@ type SignalStoreConfig = { providedIn?: 'root'; protectedState?: boolean };
 
 type SignalStoreMembers<FeatureResult extends SignalStoreFeatureResult> =
   Prettify<
-    OmitPrivate<
-      StateSignals<FeatureResult['state']> &
-        FeatureResult['props'] &
-        FeatureResult['methods']
+    Readonly<
+      OmitPrivate<
+        StateSignals<FeatureResult['state']> &
+          FeatureResult['props'] &
+          FeatureResult['methods']
+      >
     >
   >;
 
diff --git a/modules/signals/src/state-source.ts b/modules/signals/src/state-source.ts
index 4db6ec3a68..7de0ea7f07 100644
--- a/modules/signals/src/state-source.ts
+++ b/modules/signals/src/state-source.ts
@@ -14,11 +14,11 @@ const STATE_WATCHERS = new WeakMap<Signal<object>, Array<StateWatcher<any>>>();
 export const STATE_SOURCE = Symbol('STATE_SOURCE');
 
 export type WritableStateSource<State extends object> = {
-  [STATE_SOURCE]: WritableSignal<State>;
+  readonly [STATE_SOURCE]: WritableSignal<State>;
 };
 
 export type StateSource<State extends object> = {
-  [STATE_SOURCE]: Signal<State>;
+  readonly [STATE_SOURCE]: Signal<State>;
 };
 
 export type PartialStateUpdater<State extends object> = (