Lothar-Siegfried Freiherr von Richthofen (27 September 1894 – 4 July 1922) was a First World War fighter ace credited with 40 victories. He was a younger brother of top-scoring ace Manfred von Richthofen (the Red Baron).

Considering the amount of time Lothar von Richthofen spent on the front and in hospitals, he was one of the most combat efficient and prolific flying aces of the war, perhaps even more so than his brother Manfred. Of his total of 40 confirmed victories, Lothar scored 33 in just three months: 15 in April 1917, 8 in May 1917, and 10 in August 1918.

He survived the war only to die in a civilian plane crash in 1922.