Petr Alexandrovich Ryazin (born 20.10.1906) is a Soviet physicist, an academician of the Academy of Sciences of the Kirghiz SSR (1960) and its vice-president in 1960–65. Born in Solnechnogorsk (now in Moscow Region). Graduated from Moscow University (1931). In 1935–60 he worked at the Physical Institute of the USSR Academy of Sciences and at the same time in 1952–60 was professor at the Moscow Institute of Engineering Physics, and in 1965–69 deputy director of the Institute of Physics and Mathematics of the Academy of Sciences of the Kirghiz SSR.
His research was devoted to radiophysics, the theory of nonlinear oscillations and its applications, nuclear physics and accelerator technology. He established that the phase velocity of radio waves near the Earth's surface is constant. He developed (1946–60) the theory of the injection and capture of electrons in cyclic acceleration in betatrons and synchrotrons.