Royal Mail Ship (or Steamer), usually seen in its abbreviated form RMS, is the ship prefix used of any seagoing vessel that carries mail under contract to the British Royal Mail.

The designation has been used since the 1840s. It was used by a large number of companies, but is often associated in particular with the Cunard line, which held a number of high-profile mail contract business, and which traditionally prefixed the titles of all its ships with the initials "RMS". The best-known ship carrying the prefix is almost certainly the RMS Titanic, of the White Star Line.

Technically, a ship would use the prefix only while contracted to carry mail, and would revert at other times to a standard type designation such as "SS".

In recent years the shift to air transport for mail has left only a few ships with the right to the prefix; notably RMS St. Helena , which serves the island of St. Helena in the South Atlantic, and RMS Queen Mary 2.

RMMV, for Royal Mail Motor Vessel, and RMMS, for Royal Mail Motor Ship, are also sometimes seen, but are much less common.