Dirk Koster (5.10.1889-12.02.1950) was a Dutch physicist. He was born in Amsterdam. He graduated from the University of Leiden (1916). In 1916 — 20 he worked at the Higher Technical School in Delft, in 1920 —22 at the University of Lund and in 1922—23 at Copenhagen, and in 1924—49 he was professor and director of the laboratory of the University of Groningen.
His research was in the fields of X-ray spectroscopy, atomic and nuclear physics and neutron physics. Together with G. Hevesy he discovered hafnium (1923).