Abraham Plessner (February 13, 1900 – April 18, 1961) was a Russian mathematician. He was born on February 13, 1900, to a Jewish family in Łódź, which is now in Poland. He studied at secondary school where he was taught in Russian, German, and Polish. He studied at the University of Giessen where he studied under Schlesinger and Friedrich Engel. He also studied at the University of Göttingen and Berlin. He completed his doctorate from the University of Giessen in 1922. Jointly with Hensel he edited Kronecker's collected works.
After completing his doctorate, Plessner worked in Marburg where he published a paper containing what is now called Plessner's theorem. It is a theorem concerning the boundary behaviour of functions meromorphic in the unit disk.
Plessner submitted his habilitation, the extra post-doctoral qualification needed to lecture at a German university, to the faculty at the University of Giessen. He was denied because he happened to be a Russian citizen. He then moved to Moscow and joined the research group of Nikolai Nikolaevich Luzin at Moscow State University.
In 1939, he became a professor at Moscow State University. He also held a position at the Mathematical Institute of the USSR Academy of Sciences. In 1949, he was dismissed from both posts during the Soviet campaign against "rootless cosmopolitans".
Plessner is widely viewed as a founder of the Moscow school of functional analysis. He faced financial and health problems in his later years. He died on April 18, 1961, in Moscow.