diff --git a/.changeset/orange-apes-behave.md b/.changeset/orange-apes-behave.md
new file mode 100644
index 000000000..8bf2e3557
--- /dev/null
+++ b/.changeset/orange-apes-behave.md
@@ -0,0 +1,5 @@
+---
+'@guardian/commercial': minor
+---
+
+Adds 300x50 ad size to Prebid for mobile-sticky
diff --git a/src/lib/header-bidding/slot-config.spec.ts b/src/lib/header-bidding/slot-config.spec.ts
index 599a2ca73..4cbea46e9 100644
--- a/src/lib/header-bidding/slot-config.spec.ts
+++ b/src/lib/header-bidding/slot-config.spec.ts
@@ -119,7 +119,7 @@ describe('getPrebidAdSlots', () => {
 		).toEqual([
 			{
 				key: 'mobile-sticky',
-				sizes: [createAdSize(320, 50)],
+				sizes: [createAdSize(320, 50), createAdSize(300, 50)],
 			},
 		]);
 	});
diff --git a/src/lib/header-bidding/slot-config.ts b/src/lib/header-bidding/slot-config.ts
index f9a0cc9c3..15decfef0 100644
--- a/src/lib/header-bidding/slot-config.ts
+++ b/src/lib/header-bidding/slot-config.ts
@@ -188,7 +188,9 @@ const getSlots = (): HeaderBiddingSizeMapping => {
 			],
 		},
 		'mobile-sticky': {
-			mobile: shouldIncludeMobileSticky() ? [adSizes.mobilesticky] : [],
+			mobile: shouldIncludeMobileSticky()
+				? [adSizes.mobilesticky, createAdSize(300, 50)]
+				: [],
 		},
 		'crossword-banner': {
 			desktop: isCrossword ? [adSizes.leaderboard] : [],