diff --git a/src/components/layout/TopBar.tsx b/src/components/layout/TopBar.tsx
index 67e1405..b32da5a 100644
--- a/src/components/layout/TopBar.tsx
+++ b/src/components/layout/TopBar.tsx
@@ -7,7 +7,7 @@
 import { Link, useNavigate } from 'react-router-dom';
 import { useTranslation } from 'react-i18next';
 import * as LucideIcons from 'lucide-react';
-const { Sun, Moon, User, LogOut, Check, Menu, Sparkles, Search } = LucideIcons;
+const { Sun, Moon, User, LogOut, Check, Menu, Sparkles, Search, Radius } = LucideIcons;
 import { Button } from '@/components/ui/button';
 import { CommandPalette } from '@/components/common/CommandPalette';
 import {
@@ -43,6 +43,8 @@ export function TopBar() {
   const navigate = useNavigate();
   const theme = useUIStore((s) => s.theme);
   const toggleTheme = useUIStore((s) => s.toggleTheme);
+  const radius = useUIStore((s) => s.radius);
+  const toggleRadius = useUIStore((s) => s.toggleRadius);
   const toggleSidebar = useUIStore((s) => s.toggleSidebar);
   const setActiveSection = useUIStore((s) => s.setActiveSection);
   const accounts = useAuthStore((s) => s.accounts);
@@ -189,6 +191,13 @@ export function TopBar() {
               </>
             )}
 
+            <DropdownMenuItem onClick={toggleRadius}>
+              <Radius className="mr-2 h-4 w-4" />
+              {t('squareCorners', 'Square corners')}
+              {radius === 'square' && <Check className="ml-auto h-4 w-4" />}
+            </DropdownMenuItem>
+            <DropdownMenuSeparator />
+
             <DropdownMenuItem
               onClick={() => {
                 const endSessionEndpoint = useAuthStore.getState().endSessionEndpoint;
diff --git a/src/i18n/en.json b/src/i18n/en.json
index 3eeb1fd..efe6b32 100644
--- a/src/i18n/en.json
+++ b/src/i18n/en.json
@@ -337,6 +337,7 @@
     "periodValue": "Period value"
   },
   "sections": "Sections",
+  "squareCorners": "Square corners",
   "toggleTheme": "Toggle theme",
   "tracing": {
     "addFilter": "Add filter",
diff --git a/src/index.css b/src/index.css
index 67a2012..208ecdb 100644
--- a/src/index.css
+++ b/src/index.css
@@ -90,6 +90,12 @@
   --chart-5: 340 75% 60%;
 }
 
+/* Square mode: every radius token derives multiplicatively from --radius,
+   so zeroing it here flattens all corners (rounded-full stays untouched). */
+:root[data-radius='square'] {
+  --radius: 0px;
+}
+
 @theme inline {
   --color-background: var(--background);
   --color-foreground: var(--foreground);
@@ -127,10 +133,10 @@
   --color-chart-4: hsl(var(--chart-4));
   --color-chart-5: hsl(var(--chart-5));
 
-  --radius-sm: calc(var(--radius) - 4px);
-  --radius-md: calc(var(--radius) - 2px);
+  --radius-sm: calc(var(--radius) * 0.5);
+  --radius-md: calc(var(--radius) * 0.75);
   --radius-lg: var(--radius);
-  --radius-xl: calc(var(--radius) + 4px);
+  --radius-xl: calc(var(--radius) * 1.5);
 
   --animate-collapsible-down: collapsible-down 0.2s ease-out;
   --animate-collapsible-up: collapsible-up 0.2s ease-out;
diff --git a/src/stores/uiStore.ts b/src/stores/uiStore.ts
index 8bfd068..ee0d44b 100644
--- a/src/stores/uiStore.ts
+++ b/src/stores/uiStore.ts
@@ -8,14 +8,18 @@ import { create } from 'zustand';
 import { persist } from 'zustand/middleware';
 
 type Theme = 'light' | 'dark';
+type Radius = 'rounded' | 'square';
 
 interface UIState {
   theme: Theme;
+  radius: Radius;
   sidebarOpen: boolean;
   activeSection: string;
 
   toggleTheme: () => void;
   setTheme: (theme: Theme) => void;
+  toggleRadius: () => void;
+  setRadius: (radius: Radius) => void;
   toggleSidebar: () => void;
   setSidebarOpen: (open: boolean) => void;
   setActiveSection: (section: string) => void;
@@ -29,11 +33,16 @@ function applyThemeClass(theme: Theme) {
   }
 }
 
+function applyRadiusAttribute(radius: Radius) {
+  document.documentElement.dataset.radius = radius;
+}
+
 export const useUIStore = create<UIState>()(
   persist(
     (set, get) => ({
       theme:
         typeof window !== 'undefined' && window.matchMedia?.('(prefers-color-scheme: dark)').matches ? 'dark' : 'light',
+      radius: 'rounded',
       sidebarOpen: typeof window !== 'undefined' ? (window.matchMedia?.('(min-width: 768px)').matches ?? true) : true,
       activeSection: '',
 
@@ -48,6 +57,17 @@ export const useUIStore = create<UIState>()(
         set({ theme });
       },
 
+      toggleRadius: () => {
+        const next = get().radius === 'rounded' ? 'square' : 'rounded';
+        applyRadiusAttribute(next);
+        set({ radius: next });
+      },
+
+      setRadius: (radius) => {
+        applyRadiusAttribute(radius);
+        set({ radius });
+      },
+
       toggleSidebar: () => {
         set({ sidebarOpen: !get().sidebarOpen });
       },
@@ -64,11 +84,13 @@ export const useUIStore = create<UIState>()(
       name: 'stalwart-ui',
       partialize: (state) => ({
         theme: state.theme,
+        radius: state.radius,
       }),
       onRehydrateStorage: () => {
         return (state) => {
           if (state) {
             applyThemeClass(state.theme);
+            applyRadiusAttribute(state.radius ?? 'rounded');
           }
         };
       },