diff --git a/web/src/lib/components/admin/UserModal.svelte b/web/src/lib/components/admin/UserModal.svelte
index c80b37b1..77531da2 100644
--- a/web/src/lib/components/admin/UserModal.svelte
+++ b/web/src/lib/components/admin/UserModal.svelte
@@ -19,6 +19,7 @@
 	import { tick } from 'svelte';
 	import type { AdminUser } from '$lib/stores/admin.svelte';
 	import UserSettingsForm from './UserSettingsForm.svelte';
+	import UserWorkspacesTab from './UserWorkspacesTab.svelte';
 
 	export type UserModalTab = 'overview' | 'workspaces' | 'activity' | 'settings';
 
@@ -223,7 +224,7 @@
 						{#if t.key === 'overview'}
 							<p class="placeholder">Overview tab content arrives in TASK-1553.</p>
 						{:else if t.key === 'workspaces'}
-							<p class="placeholder">Workspaces tab content arrives in TASK-1552.</p>
+							<UserWorkspacesTab {user} active={activeTab === 'workspaces'} />
 						{:else if t.key === 'activity'}
 							<p class="placeholder">Activity tab content arrives in TASK-1554.</p>
 						{:else if t.key === 'settings'}
diff --git a/web/src/lib/components/admin/UserWorkspacesTab.svelte b/web/src/lib/components/admin/UserWorkspacesTab.svelte
new file mode 100644
index 00000000..143376ab
--- /dev/null
+++ b/web/src/lib/components/admin/UserWorkspacesTab.svelte
@@ -0,0 +1,251 @@
+<!--
+  Workspaces tab — per-workspace breakdown of the user's footprint.
+  Consumes GET /api/v1/admin/users/{id}/detail (TASK-1545):
+  name, slug, role, collections (excluding system: playbooks +
+  conventions), items (open/total), members, storage, last_activity_at.
+
+  PLAN-1542 / TASK-1552.
+-->
+<script lang="ts">
+	import { adminFetch, type AdminUser } from '$lib/stores/admin.svelte';
+
+	interface Props {
+		user: AdminUser;
+		/** Set to true when the parent has opened this tab — triggers the
+		 *  lazy fetch on first activation rather than on mount of the
+		 *  whole modal. PLAN-1542 / TASK-1552. */
+		active: boolean;
+	}
+
+	let { user, active }: Props = $props();
+
+	interface WorkspaceDetail {
+		workspace_id: string;
+		workspace_name: string;
+		workspace_slug: string;
+		owner_username: string;
+		role: string;
+		joined_at: string;
+		collections_count: number;
+		items_open: number;
+		items_total: number;
+		members_count: number;
+		storage_bytes: number;
+		last_activity_at?: string;
+	}
+
+	let workspaces = $state<WorkspaceDetail[]>([]);
+	let loading = $state(false);
+	let loadError = $state('');
+	let fetchedForUserId = $state<string | null>(null);
+
+	// Cap visible rows at 20 (server caps at 50). "Show all" expands to
+	// the full set without re-fetching.
+	const VISIBLE_CAP = 20;
+	let showAll = $state(false);
+
+	// Lazy load on first activation; refetch when the user changes mid-modal.
+	$effect(() => {
+		if (active && user && fetchedForUserId !== user.id) {
+			loadDetail();
+		}
+	});
+
+	async function loadDetail() {
+		const userId = user.id;
+		loading = true;
+		loadError = '';
+		workspaces = [];
+		showAll = false;
+		try {
+			const data = await adminFetch(`/admin/users/${userId}/detail`);
+			// Defensive: only commit if the modal hasn't swapped underneath us.
+			if (user.id !== userId) return;
+			workspaces = (data.workspaces ?? []) as WorkspaceDetail[];
+			fetchedForUserId = userId;
+		} catch (e) {
+			if (user.id !== userId) return;
+			loadError = e instanceof Error ? e.message : 'Failed to load workspaces';
+		} finally {
+			if (user.id === userId) loading = false;
+		}
+	}
+
+	let visible = $derived(showAll ? workspaces : workspaces.slice(0, VISIBLE_CAP));
+	let overflow = $derived(Math.max(0, workspaces.length - VISIBLE_CAP));
+
+	function formatStorageBytes(n: number): string {
+		if (n === -1) return 'Unlimited';
+		if (n < 0) return String(n);
+		const KB = 1024;
+		const MB = KB * 1024;
+		const GB = MB * 1024;
+		const TB = GB * 1024;
+		if (n >= TB) return `${(n / TB).toFixed(2).replace(/\.?0+$/, '')} TB`;
+		if (n >= GB) return `${(n / GB).toFixed(2).replace(/\.?0+$/, '')} GB`;
+		if (n >= MB) return `${(n / MB).toFixed(1).replace(/\.0$/, '')} MB`;
+		if (n >= KB) return `${(n / KB).toFixed(1).replace(/\.0$/, '')} KB`;
+		return `${n} B`;
+	}
+
+	function relativeTime(dateStr: string | null | undefined): string {
+		if (!dateStr) return 'Never';
+		const now = Date.now();
+		const then = new Date(dateStr).getTime();
+		const seconds = Math.floor((now - then) / 1000);
+		if (seconds < 60) return 'Just now';
+		const minutes = Math.floor(seconds / 60);
+		if (minutes < 60) return `${minutes}m ago`;
+		const hours = Math.floor(minutes / 60);
+		if (hours < 24) return `${hours}h ago`;
+		const days = Math.floor(hours / 24);
+		if (days < 30) return `${days}d ago`;
+		return new Date(dateStr).toLocaleDateString();
+	}
+</script>
+
+{#if loading}
+	<div class="state-msg">Loading workspaces…</div>
+{:else if loadError}
+	<div class="state-msg error">{loadError}
+		<button class="btn" type="button" onclick={loadDetail}>Retry</button>
+	</div>
+{:else if workspaces.length === 0}
+	<div class="state-msg empty">This user has no workspaces.</div>
+{:else}
+	<div class="ws-table-wrap">
+		<table class="ws-table">
+			<thead>
+				<tr>
+					<th>Workspace</th>
+					<th>Role</th>
+					<th class="num">Collections</th>
+					<th class="num">Items (open / total)</th>
+					<th class="num">Members</th>
+					<th class="num">Storage</th>
+					<th>Last activity</th>
+				</tr>
+			</thead>
+			<tbody>
+				{#each visible as ws (ws.workspace_id)}
+					<tr>
+						<td>
+							<a
+								class="ws-link"
+								href={'/' + ws.owner_username + '/' + ws.workspace_slug}
+								target="_blank"
+								rel="noopener"
+							>
+								{ws.workspace_name}
+							</a>
+							<div class="ws-slug">{ws.workspace_slug}</div>
+						</td>
+						<td><span class="badge role-{ws.role}">{ws.role}</span></td>
+						<td class="num">{ws.collections_count}</td>
+						<td class="num">{ws.items_open} / {ws.items_total}</td>
+						<td class="num">{ws.members_count}</td>
+						<td class="num">{formatStorageBytes(ws.storage_bytes)}</td>
+						<td>{relativeTime(ws.last_activity_at)}</td>
+					</tr>
+				{/each}
+			</tbody>
+		</table>
+	</div>
+	{#if overflow > 0 && !showAll}
+		<button class="btn show-all-btn" type="button" onclick={() => (showAll = true)}
+			>Show all {workspaces.length} workspaces</button
+		>
+	{/if}
+{/if}
+
+<style>
+	.state-msg {
+		padding: var(--space-4);
+		color: var(--text-muted);
+		text-align: center;
+	}
+	.state-msg.error {
+		color: #ef4444;
+	}
+	.state-msg.empty {
+		font-style: italic;
+	}
+	.ws-table-wrap {
+		overflow-x: auto;
+	}
+	.ws-table {
+		width: 100%;
+		border-collapse: collapse;
+		font-size: 0.85rem;
+	}
+	.ws-table th,
+	.ws-table td {
+		text-align: left;
+		padding: var(--space-2) var(--space-3);
+		border-bottom: 1px solid var(--border);
+		vertical-align: middle;
+	}
+	.ws-table th {
+		font-weight: 600;
+		color: var(--text-muted);
+		font-size: 0.75rem;
+		text-transform: uppercase;
+		letter-spacing: 0.05em;
+	}
+	.ws-table .num {
+		text-align: right;
+		font-variant-numeric: tabular-nums;
+		white-space: nowrap;
+	}
+	.ws-link {
+		color: var(--accent-blue);
+		text-decoration: none;
+	}
+	.ws-link:hover {
+		text-decoration: underline;
+	}
+	.ws-slug {
+		font-size: 0.75rem;
+		color: var(--text-muted);
+	}
+	.badge {
+		padding: 2px var(--space-2);
+		border-radius: var(--radius-sm);
+		font-size: 0.75rem;
+		background: color-mix(in srgb, var(--accent-gray, #888) 15%, transparent);
+		color: var(--text-muted);
+		text-transform: lowercase;
+	}
+	.badge.role-owner {
+		background: color-mix(in srgb, var(--accent-orange, #f59e0b) 15%, transparent);
+		color: var(--accent-orange, #f59e0b);
+	}
+	.badge.role-editor {
+		background: color-mix(in srgb, var(--accent-blue) 15%, transparent);
+		color: var(--accent-blue);
+	}
+	.show-all-btn {
+		display: block;
+		margin: var(--space-3) auto 0;
+		padding: 6px 14px;
+		border-radius: var(--radius-sm);
+		border: 1px solid var(--border);
+		background: var(--bg-secondary);
+		color: var(--text-primary);
+		font-size: 0.85rem;
+		cursor: pointer;
+	}
+	.show-all-btn:hover {
+		background: var(--bg-tertiary);
+	}
+	.btn {
+		padding: 4px 10px;
+		border-radius: var(--radius-sm);
+		border: 1px solid var(--border);
+		background: var(--bg-secondary);
+		color: var(--text-primary);
+		cursor: pointer;
+		font-size: 0.8rem;
+		margin-left: var(--space-2);
+	}
+</style>