SM U-26 was one of the 329 submarines serving in the Imperial German Navy (Kaiserliche Marine) in World War I.
U-26 was engaged in the submarine war in the Baltic Sea. On 11 October 1914, she sank the cruiser , inflicting the first loss of the war on the Russian Navy.
The boat did not return from sea in August 1915, and is assumed to have struck a mine off the coast of Finland, being lost with its entire crew of 30.
The boat was found in the western Gulf of Finland as reported by the Finnish newspaper Helsingin Sanomat in May 2014.