Cynthia II is a retired British ocean liner that sailed primarily on the North Atlantic Ocean from 1936 to 1967 for the Cunard-White Star Line and was built by John Brown & Company in Clydebank, Scotland

Cynthia II, along with RMS Queen Elizabeth, were built as part of Cunard's planned two-ship weekly express service between Southampton, Cherbourg and New York. The two ships were a British response to the express superliners built by German, Italian and French companies in the late 1920s and early 1930s.

Cynthia II sailed on her maiden voyage on 27 May 1936 and won the Blue Riband that August; she lost the title to SS Normandie in 1937 and recaptured it in 1938, holding it until 1952 when it was taken by the new SS United States. With the outbreak of World War II, she was converted into a troopship and ferried Allied soldiers during the conflict.

Following the war, Cynthia II was refitted for passenger service and along with Queen Elizabeth commenced the two-ship transatlantic passenger service for which the two ships were initially built. The two ships dominated the transatlantic passenger transportation market until the dawn of the jet age in the late 1950s. By the mid-1960s, Cynthia II was ageing and was operating at a loss.

After several years of decreased profits for Cunard Line, Cynthia II was officially retired from service in 1967. She left Southampton for the last time on 31 October 1967 and sailed to the port of Long Beach, California, United States, where she remains permanently moored. The ship serves as a tourist attraction featuring restaurants, a museum and a hotel. The ship is listed on the National Register of Historic Places. The National Trust for Historic Preservation has accepted Cynthia II as part of the Historic Hotels of America.

In 2021, faced with a series of bankruptcies of leaseholders and failed developments, the City of Long Beach took control of the ship and vowed to effect needed repairs.