William Otto Ernst Michaelis (19 July 1871 – 5 January 1948) was a German viceadmiral and head of the Naval Command within the Ministry of the Reichswehr in the Weimar Republic.

He entered the Imperial German Navy as a Seeoffizieranwärter in April 1889 and was promoted to Unterleutnant zur See (equivalent to ensign) in 1892.

He served at the rank of Unterleutnant zur See from 1900 to 1902 as a deck officer and commander with the 1st Torpedo Boat Division in Kiel.

He also attended Kiel's Naval Academy from 1900 to 1902, leading to service with the Reichsmarineamt (RMA) as a head of department and with the Admiralty Staff.

From 4 February 1915 through 28 January 1916 he served as chief of staff to the Commander of the High Seas Fleet, Admiral Hugo von Pohl.