diff --git a/src/utils/__tests__/activityTimeline.utils.test.ts b/src/utils/__tests__/activityTimeline.utils.test.ts
new file mode 100644
index 0000000..7febfce
--- /dev/null
+++ b/src/utils/__tests__/activityTimeline.utils.test.ts
@@ -0,0 +1,29 @@
+import { formatActivityAmount } from '@/utils/activityTimeline.utils';
+import { formatKeyPrice } from '@/utils/keyPriceDisplay.utils';
+
+describe('formatActivityAmount', () => {
+	it('formats a buy amount with a negative sign', () => {
+		const amount = 10_000_000n; // 1 XLM
+		expect(formatActivityAmount(amount, 'buy')).toBe(`-${formatKeyPrice(amount)}`);
+	});
+
+	it('formats a sell amount with a positive sign', () => {
+		const amount = 10_000_000n; // 1 XLM
+		expect(formatActivityAmount(amount, 'sell')).toBe(`+${formatKeyPrice(amount)}`);
+	});
+
+	it('formats a zero amount with a positive sign', () => {
+		const amount = 0n;
+		expect(formatActivityAmount(amount, 'sell')).toBe('+0.00 XLM');
+	});
+
+	it('formats a zero amount for buy type with a positive sign', () => {
+		const amount = 0n;
+		expect(formatActivityAmount(amount, 'buy')).toBe('+0.00 XLM');
+	});
+
+	it('formats a small amount with 4 decimal places', () => {
+		const amount = 5_000_000n; // 0.5 XLM
+		expect(formatActivityAmount(amount, 'sell')).toBe(`+${formatKeyPrice(amount)}`);
+	});
+});
diff --git a/src/utils/activityTimeline.utils.ts b/src/utils/activityTimeline.utils.ts
index 6102ca1..7c09c49 100644
--- a/src/utils/activityTimeline.utils.ts
+++ b/src/utils/activityTimeline.utils.ts
@@ -1,3 +1,5 @@
+import { formatKeyPrice } from '@/utils/keyPriceDisplay.utils';
+
 interface TimelineEntryWithTimestamp {
   timestamp?: number;
 }
@@ -35,3 +37,16 @@ export const formatDateHeader = (date: Date) => {
     });
   }
 };
+
+/**
+ * Formats an activity amount (buy/sell) with a sign prefix and XLM suffix.
+ * Buys are prefixed with '-', sells with '+'.
+ */
+export function formatActivityAmount(amount: bigint, type: 'buy' | 'sell'): string {
+	if (amount === 0n) {
+		return '+0.00 XLM';
+	}
+	const sign = type === 'buy' ? '-' : '+';
+	const formatted = formatKeyPrice(amount);
+	return `${sign}${formatted}`;
+}