thumb|Gravestone of Zoltán Bay (Gyulavári, Hungary)
Zoltán Lajos Bay (July 24, 1900 – October 4, 1992) was a Hungarian physicist, professor, and engineer who developed technologies, including tungsten lamps and microwave devices. He was the leader of the second group to observe radar echoes from the Moon (Moonbounce). From 1930, he worked at the University of Szeged as a professor of theoretical physics.
thumb|Bust and plaque commemorating Zoltán Bay in [[Újpest, Hungary. The plaque on the right lists his achievements.]]
In 1923 at Tungsram Ltd., a research laboratory was established for improving light sources, mainly electric bulbs. As a result of Bay's research, the 1983 conference of the International Weights and Measures Bureau accepted, as a standard, the definition of a meter (metre) as recommended by Bay.
In 1998, the State of Israel recognized him as among the Righteous Among the Nations and listed his name at Yad Vashem as rescuer number 6497.
A relative with the same name invented Bay radial speaker:BayZ