SS Sarnia was a passenger vessel built for the London and South Western Railway in 1910.

[2] During the First World War, she served in the Royal Navy as the armed boarding steamer HMS Sarnia.

[1] Propulsion was by two double ended marine boilers providing steam for a set of Parsons turbines driving three shafts.

The Admiralty requisitioned her during the First World War for use by the Royal Navy and reconfigured her as the armed boarding steamer HMS Sarnia.

[3] The Imperial German Navy submarine SM U-65 sank Sarnia in the Mediterranean Sea off Alexandria, Egypt, (31°58′N 30°55′E / 31.967°N 30.917°E / 31.967; 30.917) on 12 September 1918 with the loss of 53 crew.