diff --git a/src/store/app/featureFlagSettings.ts b/src/store/app/featureFlagSettings.ts
index 13a1766020..9691516512 100644
--- a/src/store/app/featureFlagSettings.ts
+++ b/src/store/app/featureFlagSettings.ts
@@ -41,6 +41,12 @@ export const featureFlagDescriptions = {
       'お気に入りチャンネル一覧を、お気に入りに登録されたチャンネルのみの木構造で表示します。',
     defaultValue: false,
     endAt: new Date('2025-12-31T23:59')
+  },
+  does_save_sidebar_expansion_state: {
+    title: 'サイドバーの展開状態を保持',
+    description: 'サイドバーが展開されているかどうかを保持します。',
+    defaultValue: false,
+    endAt: new Date('2025-12-31T23:59')
   }
 } as const satisfies Record<string, FeatureFlagDescription>
 
diff --git a/src/store/ui/mainView.ts b/src/store/ui/mainView.ts
index cd6a0d000f..86e37967c5 100644
--- a/src/store/ui/mainView.ts
+++ b/src/store/ui/mainView.ts
@@ -11,10 +11,19 @@ import type {
 import { useBrowserSettings } from '/@/store/app/browserSettings'
 import { useChannelsStore } from '/@/store/entities/channels'
 import { useUsersStore } from '/@/store/entities/users'
+import useIndexedDbValue from '/@/composables/utils/useIndexedDbValue'
+import { watch } from 'vue'
+import { useResponsiveStore } from './responsive'
+import { useFeatureFlagSettings } from '../app/featureFlagSettings'
 
 interface ViewInformationBase {
   type: string
 }
+
+type State = {
+  currentMainViewComponentState: MainViewComponentState
+}
+
 export type PrimaryViewInformation = ChannelView | ClipsView | DMView
 export type SecondaryViewInformation = QallView
 export type ViewInformation = ChannelView | QallView | ClipsView | DMView
@@ -66,13 +75,38 @@ export enum MainViewComponentState {
 export type HeaderStyle = 'default' | 'dark'
 
 const useMainViewStorePinia = defineStore('ui/mainView', () => {
+  const initialValue: State = {
+    currentMainViewComponentState: MainViewComponentState.Hidden
+  }
+
+  const [state, _restoring, restoringPromise] = useIndexedDbValue(
+    'store/ui/mainView',
+    1,
+    {},
+    initialValue
+  )
   const { lastOpenChannelName } = useBrowserSettings()
+  const { featureFlags } = useFeatureFlagSettings()
+  const { isMobile } = useResponsiveStore()
   const channelsStore = useChannelsStore()
   const usersStore = useUsersStore()
 
   const layout = ref<LayoutType>('single')
 
   const currentMainViewComponentState = ref(MainViewComponentState.Hidden)
+
+  watch(
+    () => featureFlags.value.does_save_sidebar_expansion_state.enabled,
+    async enabled => {
+      if (!isMobile.value && enabled) {
+        await restoringPromise.value
+        currentMainViewComponentState.value =
+          state.currentMainViewComponentState
+      }
+    },
+    { immediate: true }
+  )
+
   const isSidebarOpen = computed(
     () =>
       currentMainViewComponentState.value ===
@@ -86,6 +120,10 @@ const useMainViewStorePinia = defineStore('ui/mainView', () => {
     () => currentMainViewComponentState.value === MainViewComponentState.Hidden
   )
 
+  watch(currentMainViewComponentState, newState => {
+    state.currentMainViewComponentState = newState
+  })
+
   const lastScrollPosition = ref(0)
   const primaryView = ref<PrimaryViewInformation>({
     type: 'channel',