Karl Ferdinand Braun (June 6, 1850 - April 20, 1918) was a German physicist.

Born in Fulda. Educated at the University of Marburg and the University of Berlin, receiving his Ph.D. in 1872. In 1874 he discovered the point-contact rectifier effect. He became director of the Physical Institute and professor of physics at Strasbourg in 1895. In 1897 he built the first cathode-ray tube oscilloscope, the CRT is still called the "Braun tube" at the University of Karlsruhe, Germany. He also worked on wireless telegraphy from 1898, inventing the crystal rectifier and Guglielmo Marconi admitted to 'borrowing' from Braun's patents. In 1909 Braun shared the Nobel Prize for physics with Marconi for "contributions to the development of wireless telegraphy."

Braun was detained while in America because of his German citizenship when the U.S. entered WWI in 1917. He died before the war ended in 1918.