It is 8 miles (13 km) northwest of Southampton and 11 miles (18 km) south-west of Winchester. Just under 15,000 people live in Romsey, which has an area of about 4.93 square kilometres.

Romsey lies on the River Test, which is famous for fly fishing, predominantly trout. It is one of the principal towns in the Test Valley Borough. A large Norman abbey dominates the centre of the town.

Romsey was home of the late Lord Mountbatten of Burma, the 19th century British prime minister Lord Palmerston, and the 17th century philosopher and economist William Petty.

Romsey's MP has been Sandra Gidley of the Liberal Democrats since a by-election in 2000 after the previous Conservative MP Michael Colvin died with his wife in a house fire. Gidley's majority was cut to 125 votes in the 2005 General Election, possibly because she was targeted by supporters of fox hunting^.

Romsey is twinned with Paimpol in Brittany, France, and Battenberg, Germany.