HMS Mersey is a River-class offshore patrol vessel of the British Royal Navy. Named after the River Mersey, she is the fifth RN vessel to carry the name and the first to be named Mersey in 84 years. Various tenders were renamed Mersey during their service with Mersey Division Royal Naval Reserve between the early 1950s and late 1970s.

The River class is a class of offshore patrol vessels built primarily for the Royal Navy of the United Kingdom. A total of nine were built for the Royal Navy (RN), four Batch 1 and five Batch 2. One Batch 1 (HMS Clyde), which was the Falklands guard ship, was decommissioned and transferred at the end of its lease to the Royal Bahrain Naval Force.