Johann Heinrich (Harry) Diemer (7 November 1904 – June 1945) was born in Dronrijp, the Netherlands.
His father was the reverend N. Diemer, who served at the Reformed Church at Vijfhuizen.
He gave much of his free time to the Association for Calvinistic Philosophy; he was secretary to the editorial board for its journal Philosophia Reformata from its inception in 1936.
During the Second World War, he became acquainted with the young biology student Jan Lever, the later professor of zoology at the Vrije Universiteit Amsterdam.
In January 1945 Diemer was arrested by the Nazis and was sent to the Neuengamme concentration camp.