diff --git a/web/src/App.tsx b/web/src/App.tsx
index a5afdcc93..8ad767e5b 100644
--- a/web/src/App.tsx
+++ b/web/src/App.tsx
@@ -5,7 +5,7 @@ import { Outlet } from "react-router-dom";
 import useNavigateTo from "./hooks/useNavigateTo";
 import { userStore, instanceStore } from "./store";
 import { cleanupExpiredOAuthState } from "./utils/oauth";
-import { loadTheme } from "./utils/theme";
+import { loadTheme, setupSystemThemeListener } from "./utils/theme";
 
 const App = observer(() => {
   const { i18n } = useTranslation();
@@ -85,6 +85,25 @@ const App = observer(() => {
     }
   }, [userGeneralSetting?.theme, instanceStore.state.theme]);
 
+  // Listen for system theme changes when using "system" theme
+  useEffect(() => {
+    const currentTheme = userGeneralSetting?.theme || instanceStore.state.theme;
+
+    // Only set up listener if theme is "system"
+    if (currentTheme !== "system") {
+      return;
+    }
+
+    // Set up listener for OS theme preference changes
+    const cleanup = setupSystemThemeListener(() => {
+      // Reload theme when system preference changes
+      loadTheme(currentTheme);
+    });
+
+    // Cleanup listener on unmount or when theme changes
+    return cleanup;
+  }, [userGeneralSetting?.theme, instanceStore.state.theme]);
+
   return <Outlet />;
 });
 
diff --git a/web/src/components/Settings/PreferencesSection.tsx b/web/src/components/Settings/PreferencesSection.tsx
index 5df3f42ad..3d1353d75 100644
--- a/web/src/components/Settings/PreferencesSection.tsx
+++ b/web/src/components/Settings/PreferencesSection.tsx
@@ -27,6 +27,9 @@ const PreferencesSection = observer(() => {
   };
 
   const handleThemeChange = async (theme: string) => {
+    // Update instance store immediately for instant UI feedback
+    instanceStore.state.setPartial({ theme });
+    // Persist to user settings
     await userStore.updateUserGeneralSetting({ theme }, ["theme"]);
   };
 
@@ -34,7 +37,7 @@ const PreferencesSection = observer(() => {
   const setting: UserSetting_GeneralSetting = generalSetting || {
     locale: "en",
     memoVisibility: "PRIVATE",
-    theme: "default",
+    theme: "system",
   };
 
   return (
diff --git a/web/src/components/ThemeSelect.tsx b/web/src/components/ThemeSelect.tsx
index d9f2fd1bc..623a5d04c 100644
--- a/web/src/components/ThemeSelect.tsx
+++ b/web/src/components/ThemeSelect.tsx
@@ -1,4 +1,4 @@
-import { Moon, Palette, Sun, Wallpaper } from "lucide-react";
+import { Moon, Monitor, Palette, Sun, Wallpaper } from "lucide-react";
 import { Select, SelectContent, SelectItem, SelectTrigger, SelectValue } from "@/components/ui/select";
 import { instanceStore } from "@/store";
 import { THEME_OPTIONS } from "@/utils/theme";
@@ -10,6 +10,7 @@ interface ThemeSelectProps {
 }
 
 const THEME_ICONS: Record<string, JSX.Element> = {
+  system: <Monitor className="w-4 h-4" />,
   default: <Sun className="w-4 h-4" />,
   "default-dark": <Moon className="w-4 h-4" />,
   paper: <Palette className="w-4 h-4" />,
@@ -17,7 +18,7 @@ const THEME_ICONS: Record<string, JSX.Element> = {
 };
 
 const ThemeSelect = ({ value, onValueChange, className }: ThemeSelectProps = {}) => {
-  const currentTheme = value || instanceStore.state.theme || "default";
+  const currentTheme = value || instanceStore.state.theme || "system";
 
   const handleThemeChange = (newTheme: Theme) => {
     if (onValueChange) {
diff --git a/web/src/store/instance.ts b/web/src/store/instance.ts
index b02a8ae4a..06787913a 100644
--- a/web/src/store/instance.ts
+++ b/web/src/store/instance.ts
@@ -17,7 +17,7 @@ import { createRequestKey } from "./store-utils";
 /**
  * Valid theme options
  */
-const VALID_THEMES = ["default", "default-dark", "paper", "whitewall"] as const;
+const VALID_THEMES = ["system", "default", "default-dark", "paper", "whitewall"] as const;
 export type Theme = (typeof VALID_THEMES)[number];
 
 /**
@@ -40,7 +40,7 @@ class InstanceState extends StandardState {
    * Current theme
    * Note: Accepts string for flexibility, but validates to Theme
    */
-  theme: Theme | string = "default";
+  theme: Theme | string = "system";
 
   /**
    * Instance profile containing owner and metadata
@@ -249,7 +249,7 @@ export const initialInstanceStore = async (): Promise<void> => {
     const instanceGeneralSetting = instanceStore.state.generalSetting;
     instanceStore.state.setPartial({
       locale: instanceGeneralSetting.customProfile?.locale || "en",
-      theme: "default",
+      theme: instanceGeneralSetting.theme || "system",
       profile: instanceProfile,
     });
   } catch (error) {
@@ -257,7 +257,7 @@ export const initialInstanceStore = async (): Promise<void> => {
     // Set default fallback values
     instanceStore.state.setPartial({
       locale: "en",
-      theme: "default",
+      theme: "system",
     });
   }
 };
diff --git a/web/src/types/modules/setting.d.ts b/web/src/types/modules/setting.d.ts
index 456c428a8..ffe65b304 100644
--- a/web/src/types/modules/setting.d.ts
+++ b/web/src/types/modules/setting.d.ts
@@ -1 +1 @@
-type Theme = "default" | "default-dark" | "paper" | "whitewall";
+type Theme = "system" | "default" | "default-dark" | "paper" | "whitewall";
diff --git a/web/src/utils/theme.ts b/web/src/utils/theme.ts
index e4e80a2e0..d270c3a79 100644
--- a/web/src/utils/theme.ts
+++ b/web/src/utils/theme.ts
@@ -2,10 +2,11 @@ import defaultDarkThemeContent from "../themes/default-dark.css?raw";
 import paperThemeContent from "../themes/paper.css?raw";
 import whitewallThemeContent from "../themes/whitewall.css?raw";
 
-const VALID_THEMES = ["default", "default-dark", "paper", "whitewall"] as const;
+const VALID_THEMES = ["system", "default", "default-dark", "paper", "whitewall"] as const;
 type ValidTheme = (typeof VALID_THEMES)[number];
 
 const THEME_CONTENT: Record<ValidTheme, string | null> = {
+  system: null, // System theme dynamically chooses between default and default-dark
   default: null,
   "default-dark": defaultDarkThemeContent,
   paper: paperThemeContent,
@@ -18,8 +19,9 @@ export interface ThemeOption {
 }
 
 export const THEME_OPTIONS: ThemeOption[] = [
-  { value: "default", label: "Default Light" },
-  { value: "default-dark", label: "Default Dark" },
+  { value: "system", label: "Sync with system" },
+  { value: "default", label: "Light" },
+  { value: "default-dark", label: "Dark" },
   { value: "paper", label: "Paper" },
   { value: "whitewall", label: "Whitewall" },
 ];
@@ -38,6 +40,18 @@ export const getSystemTheme = (): "default" | "default-dark" => {
   return "default";
 };
 
+/**
+ * Resolves the actual theme to apply based on user preference
+ * If theme is "system", returns the system preference, otherwise returns the theme as-is
+ */
+export const resolveTheme = (theme: string): "default" | "default-dark" | "paper" | "whitewall" => {
+  if (theme === "system") {
+    return getSystemTheme();
+  }
+  const validTheme = validateTheme(theme);
+  return validTheme === "system" ? getSystemTheme() : validTheme;
+};
+
 /**
  * Gets the theme that should be applied on initial load
  * Priority: stored user preference -> system preference -> default
@@ -53,8 +67,8 @@ export const getInitialTheme = (): ValidTheme => {
     // localStorage might not be available
   }
 
-  // Fall back to system preference
-  return getSystemTheme();
+  // Fall back to system preference (return "system" to enable auto-switching)
+  return "system";
 };
 
 /**
@@ -68,12 +82,15 @@ export const applyThemeEarly = (): void => {
 export const loadTheme = (themeName: string): void => {
   const validTheme = validateTheme(themeName);
 
+  // Resolve "system" to actual theme based on OS preference
+  const resolvedTheme = resolveTheme(validTheme);
+
   // Remove existing theme
   document.getElementById("instance-theme")?.remove();
 
   // Apply theme (skip for default)
-  if (validTheme !== "default") {
-    const css = THEME_CONTENT[validTheme];
+  if (resolvedTheme !== "default") {
+    const css = THEME_CONTENT[resolvedTheme];
     if (css) {
       const style = document.createElement("style");
       style.id = "instance-theme";
@@ -82,13 +99,44 @@ export const loadTheme = (themeName: string): void => {
     }
   }
 
-  // Set data attribute
-  document.documentElement.setAttribute("data-theme", validTheme);
+  // Set data attribute with resolved theme
+  document.documentElement.setAttribute("data-theme", resolvedTheme);
 
-  // Store theme preference for future loads
+  // Store theme preference (original, not resolved) for future loads
   try {
     localStorage.setItem("memos-theme", validTheme);
   } catch {
     // localStorage might not be available
   }
 };
+
+/**
+ * Sets up a listener for system theme preference changes
+ * Returns a cleanup function to remove the listener
+ */
+export const setupSystemThemeListener = (onThemeChange: () => void): (() => void) => {
+  if (typeof window === "undefined" || !window.matchMedia) {
+    return () => {}; // No-op cleanup
+  }
+
+  const mediaQuery = window.matchMedia("(prefers-color-scheme: dark)");
+
+  // Handle theme change
+  const handleChange = () => {
+    onThemeChange();
+  };
+
+  // Modern API (addEventListener)
+  if (mediaQuery.addEventListener) {
+    mediaQuery.addEventListener("change", handleChange);
+    return () => mediaQuery.removeEventListener("change", handleChange);
+  }
+
+  // Legacy API (addListener) - for older browsers
+  if (mediaQuery.addListener) {
+    mediaQuery.addListener(handleChange);
+    return () => mediaQuery.removeListener(handleChange);
+  }
+
+  return () => {}; // No-op cleanup
+};