Ivan Supek (8 April 1915 – 5 March 2007) was a Croatian physicist, philosopher, writer, playwright, peace activist and humanist.

After finishing grammar school in Zagreb in 1934, he continued pursuing his education in Vienna for a brief period, then moved to Zürich studying mathematics, physics, biology and philosophy.

Immediately after being released, instead of returning to Leipzig, he went back to Independent State of Croatia and joined the communist antifascist Yugoslav Partisans movement.

Supek was a proponent of total and unconditional nuclear disarmament, having already in 1944, fourteen months before the bombing of Hiroshima warned on the danger of misuse of atomic energy.

He was excluded from it in 1958 due to his disagreement with the Yugoslav Federal Commission for Nuclear Energy and his unwillingness to participate in a project for building the atomic bomb (an idea Josip Broz Tito himself did not like much, and which was subsequently abandoned).

In one of his last interviews in March 2006 [5] Supek spoke about the famous and controversial meeting between Werner Heisenberg and Niels Bohr in Copenhagen in September 1941.

In his interview, Supek claimed that the main figure of the meeting was neither Heisenberg nor Bohr but Carl Friedrich von Weizsäcker.

Von Weizsäcker's idea, probably originating from his father who was Ribbentrop's deputy, was to persuade Bohr to mediate for peace between Great Britain and Germany."

In a 1997 "open letter" which he also read on the national television and published in all the major dailies, president Tuđman accused Supek, then President of the Academy of Arts and Sciences, of supporting forces that allegedly plotted his assassination after Supek made public statements critical of presidential policies: he called for Tuđman to submit to public scrutiny his financial assets before and after the war.

Beside his scientific and humanist work, Supek wrote numerous novels and plays, with themes spanning from philosophy, science fiction to politics.