Fritz Weigert (18 September 1876 – 13 April 1947) was a German physical chemist. Weigert has made major contributions in the field of photochemistry. He was born in Berlin. He was the nephew of both Karl Weigert and Paul Ehrlich. He was married to Margarete Behmer. Around 1908, he began teaching and conducting research at Berlin University - after studying there. He was a photochemistry professor at Leipzig University from 1914 until being, like other Jewish scientists, forced out by the Nazis in 1934. On January 1, 1935, he immigrated to England and in 1936 was director of the Physiochemical Department of the Cancer Research Institute at Mount Vernon Hospital, Northwood.
He elucidated the metabolism of the carcinogenic hydrocarbon 3,4-benzpyrene by fluorescence spectroscopy.