HMHS Britannic () was the third and final vessel of the White Star Line's Olympic class of steamships and the second White Star ship to bear the name Britannic. She was the youngest sister of the RMS Olympic and the RMS Titanic and was intended to enter service as a transatlantic passenger liner. She was operated as a hospital ship from 1915 until her sinking near the Greek island of Kea, in the Aegean Sea, in 1916.

Britannic was launched just before the start of the First World War. She was designed to be the safest of the three ships with design changes made during construction due to lessons learned from the sinking of the Titanic. She was laid up at her builders, Harland and Wolff, in Belfast for many months before being requisitioned as a hospital ship. In 1915 and 1916 she served between the United Kingdom and the Dardanelles. On the morning of 21 November 1916 she was shaken by an explosion caused by a naval mine of the Imperial German Navy near the Greek island of Kea and sank 55 minutes later, killing 30 people.

There were 1,066 people on board; the 1,036 survivors were rescued from the water and lifeboats. Britannic was the largest ship lost in the First World War.After the First World War the White Star Line was compensated for the loss of Britannic by the award of SS Bismarck as part of postwar reparations; she entered service as RMS Majestic.

The wreck was located and explored by Jacques-Yves Cousteau in 1975. The vessel is the largest intact passenger ship on the seabed.