Hay served at the Siege of San Sebastián, where Leith and Oswald, the senior officers of the 5th Division were wounded, thus giving Hay command once more. Hay therefore, on 7 October 1813, led the 5th Division at the Battle of the Bidasoa when Wellington's army overran the French lines and gained a foothold in France. Hay's troops were the first to cross the river, wading across at first light and surprising the French who did not realise the water was only chest deep at low tide.