diff --git a/public/theme.js b/public/theme.js
index 67f76e9a..7eb9289c 100644
--- a/public/theme.js
+++ b/public/theme.js
@@ -11,9 +11,13 @@
   var theme = 'dark';
   try {
     var cfg = JSON.parse(localStorage.getItem('nostr:app-config') || '{}');
-    if (cfg.theme && themes[cfg.theme]) theme = cfg.theme;
+    if (cfg.theme && (themes[cfg.theme] || cfg.theme === 'system')) theme = cfg.theme;
   } catch (e) {}
-  var t = themes[theme];
+  // Resolve "system" to light or dark based on OS preference
+  if (theme === 'system') {
+    theme = window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
+  }
+  var t = themes[theme] || themes.dark;
   document.documentElement.className = theme;
   document.body.style.background = t.bg;
   var p = document.getElementById('preloader');
diff --git a/src/components/AppProvider.tsx b/src/components/AppProvider.tsx
index 88f0b28f..69970f45 100644
--- a/src/components/AppProvider.tsx
+++ b/src/components/AppProvider.tsx
@@ -2,7 +2,7 @@ import { ReactNode, useEffect } from 'react';
 import { z } from 'zod';
 import { useLocalStorage } from '@/hooks/useLocalStorage';
 import { AppContext, type AppConfig, type AppContextType, type Theme, type RelayMetadata } from '@/contexts/AppContext';
-import { themes, buildThemeCss } from '@/themes';
+import { themes, buildThemeCss, resolveTheme } from '@/themes';
 import { ThemeSchema, FeedSettingsSchema, ContentWarningPolicySchema } from '@/lib/schemas';
 
 interface AppProviderProps {
@@ -101,21 +101,35 @@ export function AppProvider(props: AppProviderProps) {
 
 /**
  * Hook to apply theme changes to the document root via an injected <style> tag.
+ * When theme is "system", resolves to "light" or "dark" based on OS preference
+ * and listens for changes to prefers-color-scheme.
  */
 function useApplyTheme(theme: Theme) {
   useEffect(() => {
-    const tokens = themes[theme] ?? themes.dark;
-    const css = buildThemeCss(tokens);
+    function apply() {
+      const resolved = resolveTheme(theme);
+      const tokens = themes[resolved] ?? themes.dark;
+      const css = buildThemeCss(tokens);
 
-    let el = document.getElementById('theme-vars') as HTMLStyleElement | null;
-    if (!el) {
-      el = document.createElement('style');
-      el.id = 'theme-vars';
-      document.head.appendChild(el);
+      let el = document.getElementById('theme-vars') as HTMLStyleElement | null;
+      if (!el) {
+        el = document.createElement('style');
+        el.id = 'theme-vars';
+        document.head.appendChild(el);
+      }
+      el.textContent = css;
+      // Now that CSS variables are set, the inline body background from
+      // theme.js is no longer needed — bg-background will resolve correctly.
+      document.body.removeAttribute('style');
+    }
+
+    apply();
+
+    // When theme is "system", listen for OS color scheme changes
+    if (theme === 'system') {
+      const mq = window.matchMedia('(prefers-color-scheme: dark)');
+      mq.addEventListener('change', apply);
+      return () => mq.removeEventListener('change', apply);
     }
-    el.textContent = css;
-    // Now that CSS variables are set, the inline body background from
-    // theme.js is no longer needed — bg-background will resolve correctly.
-    document.body.removeAttribute('style');
   }, [theme]);
 }
\ No newline at end of file
diff --git a/src/components/InitialSyncGate.tsx b/src/components/InitialSyncGate.tsx
index a117fd30..f6868aac 100644
--- a/src/components/InitialSyncGate.tsx
+++ b/src/components/InitialSyncGate.tsx
@@ -209,7 +209,8 @@ function SyncScreen({ phase }: { phase: SyncPhase }) {
 // Setup Questionnaire
 // ---------------------------------------------------------------------------
 
-const THEMES: { value: Theme; label: string; description: string; preview: string }[] = [
+const THEMES: { value: Theme; label: string; description: string; preview: string; splitPreview?: boolean }[] = [
+  { value: 'system', label: 'System', description: 'Matches your device', preview: '', splitPreview: true },
   { value: 'dark', label: 'Dark', description: 'Deep purple dark theme', preview: 'bg-[hsl(228,20%,10%)]' },
   { value: 'light', label: 'Light', description: 'Clean and bright', preview: 'bg-white border border-border' },
   { value: 'black', label: 'Black', description: 'True OLED black', preview: 'bg-black' },
@@ -963,12 +964,19 @@ function ThemeStep({
                 : 'ring-1 ring-border',
             )}
           >
-            <div
-              className={cn(
-                'w-14 h-14 rounded-full transition-transform duration-200 group-hover:scale-110',
-                theme.preview,
-              )}
-            />
+            {theme.splitPreview ? (
+              <div className="w-14 h-14 rounded-full overflow-hidden transition-transform duration-200 group-hover:scale-110 flex">
+                <div className="w-1/2 h-full bg-white" />
+                <div className="w-1/2 h-full bg-[hsl(228,20%,10%)]" />
+              </div>
+            ) : (
+              <div
+                className={cn(
+                  'w-14 h-14 rounded-full transition-transform duration-200 group-hover:scale-110',
+                  theme.preview,
+                )}
+              />
+            )}
             <div className="space-y-0.5 text-center">
               <p className="text-sm font-medium">{theme.label}</p>
               <p className="text-xs text-muted-foreground">{theme.description}</p>
diff --git a/src/components/LeftSidebar.tsx b/src/components/LeftSidebar.tsx
index 103760aa..a7de1bd2 100644
--- a/src/components/LeftSidebar.tsx
+++ b/src/components/LeftSidebar.tsx
@@ -1,6 +1,6 @@
 import { useState, useMemo, useCallback } from 'react';
 import { Link, useLocation, useNavigate } from 'react-router-dom';
-import { Home, Bell, Search, Clapperboard, BarChart3, Palette, PartyPopper, Radio, FileText, User, Settings, Bookmark, UserPlus, LogOut, Check, Moon, Sun, Heart, ChevronDown, Plus } from 'lucide-react';
+import { Home, Bell, Search, Clapperboard, BarChart3, Palette, PartyPopper, Radio, FileText, User, Settings, Bookmark, UserPlus, LogOut, Check, Moon, Sun, Heart, Monitor, ChevronDown, Plus } from 'lucide-react';
 import { Skeleton } from '@/components/ui/skeleton';
 import { ChestIcon } from '@/components/icons/ChestIcon';
 import { CardsIcon } from '@/components/icons/CardsIcon';
@@ -151,6 +151,7 @@ export function LeftSidebar() {
   };
 
   const themes: { value: Theme; label: string; icon: React.ReactNode }[] = [
+    { value: 'system', label: 'System', icon: <Monitor className="size-4" /> },
     { value: 'dark', label: 'Dark', icon: <Palette className="size-4" /> },
     { value: 'light', label: 'Light', icon: <Sun className="size-4" /> },
     { value: 'black', label: 'Black', icon: <Moon className="size-4" /> },
diff --git a/src/components/MobileDrawer.tsx b/src/components/MobileDrawer.tsx
index 0bd159ed..9fac6f68 100644
--- a/src/components/MobileDrawer.tsx
+++ b/src/components/MobileDrawer.tsx
@@ -1,5 +1,5 @@
 import { Link, useNavigate } from 'react-router-dom';
-import { User, Bookmark, Settings, LogOut, ChevronDown, ChevronUp, Sun, Moon, Heart, Clapperboard, BarChart3, Palette, PartyPopper, Radio, FileText } from 'lucide-react';
+import { User, Bookmark, Settings, LogOut, ChevronDown, ChevronUp, Sun, Moon, Heart, Monitor, Clapperboard, BarChart3, Palette, PartyPopper, Radio, FileText } from 'lucide-react';
 import { Avatar, AvatarImage, AvatarFallback } from '@/components/ui/avatar';
 import { Sheet, SheetContent, SheetTitle } from '@/components/ui/sheet';
 import { Separator } from '@/components/ui/separator';
@@ -78,6 +78,7 @@ export function MobileDrawer({ open, onOpenChange }: MobileDrawerProps) {
   }, [feedSettings]);
 
   const themes: { value: Theme; label: string; icon: React.ReactNode }[] = [
+    { value: 'system', label: 'System', icon: <Monitor className="size-5" /> },
     { value: 'dark', label: 'Dark', icon: <Palette className="size-5" /> },
     { value: 'light', label: 'Light', icon: <Sun className="size-5" /> },
     { value: 'black', label: 'Black', icon: <Moon className="size-5" /> },
diff --git a/src/components/ThemeSelector.tsx b/src/components/ThemeSelector.tsx
index c7cee8bd..bf5e50ac 100644
--- a/src/components/ThemeSelector.tsx
+++ b/src/components/ThemeSelector.tsx
@@ -11,6 +11,7 @@ interface ThemeOption {
 }
 
 const themeOptions: ThemeOption[] = [
+  { id: 'system', label: 'System' },
   { id: 'light', label: 'Light', tokens: themes.light },
   { id: 'dark', label: 'Dark', tokens: themes.dark },
   { id: 'black', label: 'Black', tokens: themes.black },
@@ -30,6 +31,74 @@ export function ThemeSelector() {
     <div className="space-y-3">
       <div className="grid grid-cols-2 gap-3 sm:grid-cols-3">
         {themeOptions.map((option) => {
+          if (option.id === 'system') {
+            const isActive = theme === 'system';
+            const lightTokens = themes.light;
+            const darkTokens = themes.dark;
+
+            return (
+              <button
+                key="system"
+                className={cn(
+                  'relative group rounded-xl border-2 p-1 transition-all focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring',
+                  isActive
+                    ? 'border-primary shadow-sm'
+                    : 'border-border hover:border-primary/40',
+                )}
+                onClick={() => setTheme('system')}
+              >
+                {/* Split preview: left light, right dark */}
+                <div className="aspect-[4/3] rounded-lg overflow-hidden relative">
+                  {/* Light half */}
+                  <div
+                    className="absolute inset-0 w-1/2"
+                    style={{ backgroundColor: hsl(lightTokens.background) }}
+                  >
+                    <div className="h-2.5 w-full" style={{ backgroundColor: hsl(lightTokens.card) }} />
+                    <div className="p-1.5 space-y-1">
+                      <div className="h-1 w-3/4 rounded-full" style={{ backgroundColor: hsl(lightTokens.foreground), opacity: 0.6 }} />
+                      <div className="h-1 w-1/2 rounded-full" style={{ backgroundColor: hsl(lightTokens.mutedForeground), opacity: 0.4 }} />
+                      <div className="pt-0.5">
+                        <div className="h-2 w-8 rounded-sm" style={{ backgroundColor: hsl(lightTokens.primary) }} />
+                      </div>
+                    </div>
+                  </div>
+                  {/* Dark half */}
+                  <div
+                    className="absolute inset-0 left-1/2"
+                    style={{ backgroundColor: hsl(darkTokens.background) }}
+                  >
+                    <div className="h-2.5 w-full" style={{ backgroundColor: hsl(darkTokens.card) }} />
+                    <div className="p-1.5 space-y-1">
+                      <div className="h-1 w-3/4 rounded-full" style={{ backgroundColor: hsl(darkTokens.foreground), opacity: 0.6 }} />
+                      <div className="h-1 w-1/2 rounded-full" style={{ backgroundColor: hsl(darkTokens.mutedForeground), opacity: 0.4 }} />
+                      <div className="pt-0.5">
+                        <div className="h-2 w-8 rounded-sm" style={{ backgroundColor: hsl(darkTokens.primary) }} />
+                      </div>
+                    </div>
+                  </div>
+
+                  {/* Active check mark */}
+                  {isActive && (
+                    <div className="absolute top-1 left-1 size-4 rounded-full flex items-center justify-center"
+                      style={{ backgroundColor: hsl(lightTokens.primary) }}
+                    >
+                      <Check className="size-2.5" style={{ color: hsl(lightTokens.primaryForeground) }} />
+                    </div>
+                  )}
+                </div>
+
+                {/* Label */}
+                <p className={cn(
+                  'mt-1.5 text-xs font-medium text-center transition-colors',
+                  isActive ? 'text-foreground' : 'text-muted-foreground',
+                )}>
+                  {option.label}
+                </p>
+              </button>
+            );
+          }
+
           if (option.id === 'custom') {
             return (
               <button
diff --git a/src/contexts/AppContext.ts b/src/contexts/AppContext.ts
index f0ef7766..3221bf2e 100644
--- a/src/contexts/AppContext.ts
+++ b/src/contexts/AppContext.ts
@@ -1,6 +1,10 @@
 import { createContext } from "react";
 
-export type Theme = "dark" | "light" | "black" | "pink";
+/** A concrete theme with its own color palette. */
+export type ConcreteTheme = "dark" | "light" | "black" | "pink";
+
+/** User-facing theme preference. "system" resolves to "light" or "dark" based on OS preference. */
+export type Theme = ConcreteTheme | "system";
 
 /**
  * How to handle events with a NIP-36 content-warning tag.
diff --git a/src/lib/schemas.ts b/src/lib/schemas.ts
index 94dbf4f7..02f1aa9b 100644
--- a/src/lib/schemas.ts
+++ b/src/lib/schemas.ts
@@ -3,7 +3,7 @@ import { z } from 'zod';
 import type { Theme, ContentWarningPolicy } from '@/contexts/AppContext';
 
 /** Zod schema for Theme validation */
-export const ThemeSchema = z.enum(['dark', 'light', 'black', 'pink']) satisfies z.ZodType<Theme>;
+export const ThemeSchema = z.enum(['dark', 'light', 'black', 'pink', 'system']) satisfies z.ZodType<Theme>;
 
 /** Zod schema for ContentWarningPolicy validation */
 export const ContentWarningPolicySchema = z.enum(['blur', 'hide', 'show']) satisfies z.ZodType<ContentWarningPolicy>;
diff --git a/src/themes.ts b/src/themes.ts
index 9858f190..86da6e91 100644
--- a/src/themes.ts
+++ b/src/themes.ts
@@ -1,4 +1,4 @@
-import type { Theme } from '@/contexts/AppContext';
+import type { ConcreteTheme, Theme } from '@/contexts/AppContext';
 
 export interface ThemeTokens {
   background: string;
@@ -30,7 +30,7 @@ export interface ThemeTokens {
   sidebarRing: string;
 }
 
-export const themes: Record<Theme, ThemeTokens> = {
+export const themes: Record<ConcreteTheme, ThemeTokens> = {
   light: {
     background: '0 0% 100%',
     foreground: '222.2 84% 4.9%',
@@ -164,3 +164,11 @@ export function buildThemeCss(tokens: ThemeTokens): string {
     .join(' ');
   return `:root { ${vars} }`;
 }
+
+/** Resolves a theme preference to a concrete theme. "system" maps to "light" or "dark" based on OS preference. */
+export function resolveTheme(theme: Theme): ConcreteTheme {
+  if (theme === 'system') {
+    return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
+  }
+  return theme;
+}