Masatoshi Koshiba (born on September 19, 1926 in Toyohashi, Aichi) is a Japanese physicist. He graduated from the University of Tokyo and received a Ph.D. in physics at the University of Rochester.

He jointly won the Nobel Prize in Physics in 2002 together with Raymond Davis and Riccardo Giacconi for their work in astrophysics, including their work for the detection of cosmic neutrinos.

According to Wikipedia:

Koshiba's award-winning work centred on neutrinos, subatomic particles that had long perplexed scientists. Since the 1920s it had been suspected that the Sun shines because of nuclear fusion reactions that transform hydrogen into helium and release energy. Later, theoretical calculations indicated that countless neutrinos must be released in these reactions and, consequently, that Earth must be exposed to a constant flood of solar neutrinos. Because neutrinos interact weakly with matter, however, only one in a trillion is stopped on its way to Earth. Neutrinos thus developed a reputation as being undetectable.

In 2003, he was awarded the Benjamin Franklin Medal in Physics.