Seven ships of the Royal Navy have been named HMS Monmouth, after the Welsh town; the name also recognises James Scott, 1st Duke of Monmouth, the "Black Duke", in whose honour the current ship flies a black flag from the mainmast.

• The first Monmouth was an eight-gun yacht built in 1666.
• The second Monmouth was a 66-gun third-rate built in 1667, at Chatham Dockyard, and broken up after a long service in 1767.
• The third Monmouth was a 64-gun third-rate built at Plymouth between 1767 and 1772; in 1796 she was made a prison ship and named Captivity , and broken up in 1818.
• The fourth Monmouth was a 64-gun third-rate built on the Thames in 1796, laid up in 1815 and broken up in 1834.
• The fifth Monmouth was a fifth-rate launched as Horspar at Plymouth in 1828, renamed Monmouth in 1868, and finally sold in 1902 after seeing use as a chapel at Devonport.
• The sixth Monmouth was the name-ship of her class of armoured cruisers of 9,800 tons displacement. Built in 1901.
• The seventh and current Monmouth is a Type 23 frigate, built at Yarrow Shipbuilders on the river Clyde, and launched on 23 November 1991.