Nikolay Nikolayevich Semyonov (April 15 (April 3, Old Style), 1896 – September 25, 1986) was a Russian/Soviet physicist and chemist. In 1956, he was awarded the Nobel Prize in Chemistry (together with Sir Cyril Norman Hinshelwood) for this work. Semyonov also became a Hero of Socialist Labor twice, received two Stalin Prizes, five Orders of Lenin, and many other awards.

Semyonov was born in Saratov and graduated from the department of physics of Petrograd University (1913–1917), where he was a student of Abram Fyodorovich Ioffe. In 1918, he moved to Samara, where he was enlisted into Kolchak's White Army during Russian Civil War.
In 1920, he returned to Petrograd and took charge of the electron phenomena laboratory of the Petrograd Physico-Technical Institute.
Semyonov, together with Pyotr Kapitsa, discovered a way to measure the magnetic field of an atomic nucleus (1922). Later the experimental setup was improved by Otto Stern and Walther Gerlach and became known as Stern-Gerlach experiment.
In 1925, Semyonov, together with Yakov Frenkel, studied kinetics of condensation and adsorption of vapors. In 1927, he studied ionisation in gases and published an important book, Chemistry of the Electron. In 1928, he, together with Vladimir Fock, created a theory of thermal disruptive discharge of dielectrics.
He lectured at the Petrograd Polytechnical Institute and was appointed Professor in 1928. In 1931, he organized the Institute of Chemical Physics of the U.S.S.R. Academy of Sciences (which has moved to Chernogolovka in 1943) and became its first director. In 1932, he became a full member of the Soviet Academy of Sciences.