Dirk Coster (5 October 1889 &ndash; 12 February 1950) was a Dutch physicist. He was a professor of Physics and Meteorology at the University of Groningen.
Coster was born in Amsterdam. On 26 February 1919 he married Lina Maria "Miep" Wijsman, who held a degree in Oriental languages.  Eventually, she was one of the first women to obtain a doctorate degree in this field from the University of Leiden.  Dirk and Miep had two sons and two daughters (Hendrik, Ada, Els, and Herman).  Coster is known as the co-discoverer of hafnium (element 72) in 1923, along with George de Hevesy, by means of X-ray spectroscopic analysis of zirconium ore. The discovery took place in Copenhagen, Denmark. Its name is derived from Hafnia, the Latin name for Copenhagen.