Promoted to rear-admiral on 4 July 1853, Dundas was appointed Third Naval Lord in the Aberdeen ministry in January 1853 and then Second Naval Lord in the same ministry in June 1854. He was appointed to the command of the Baltic Fleet in the Baltic Sea, hoisting his flag in the first-rate HMS "Duke of Wellington", in succession to James Dundas in February 1855. Dundas led the naval support during the latter stages of the Crimean War enforcing a strict blockade in the Baltic. Dundas was hesitant but the French admiral Charles Penaud persuaded him to attack the Russian fleet in the harbour of Sweaborg on 9–10 August 1855. The British provided most of the attacking force. Much of the bombardment was done with smaller vessels that the shore batteries found hard to hit. Six Russians ships of the line and 17 smaller warships were destroyed, and the Russians suffered 2,000 casualties. The allies suffered one death, 15 wounded, and damage to only one British sloop.