Theodor V. Ionescu, Prof. Dr. Doc. (born February 8, 1899, Dorohoi, Botoşani County - died 7 November 1988, Bucharest) was a Romanian physicist and inventor who made remarkable discoveries in plasma physics, ionosphere physics, ion coupling electrons in dense plasmas, masers, magnetron amplifiers, and Zeeman effects related to controlled nuclear fusion and quantum emission mechanisms in hot plasmas. Member of the Romanian Academy since December 21, 1935.

He received his Ph.D. in plasma physics first in Paris, and then in Iaşi, Romania. Thus, the history of plasma physics in Romania began in 1923 with the defense of the first PhD thesis in physics at the University of Iaşi by Theodor V. Ionescu, under the guidance of Professor Petru Bogdan. Th. V. Ionescu carried out the first experimental studies in Romania of the physics of ionized gases/plasmas.

In 1925 invented a microphone based on thermionic currents (currents emitted by heated bodies) and a light projector using the interference phenomenon.