Zacharias "Zak" Kostopoulos (Greek: Ζαχαρίας «Ζακ» Κωστόπουλος; 22 August 1985 – 21 September 2018) was a Greek-American activist, defending the rights of LGBT people, HIV-positive people, sex workers and refugees. He was also a drag performer under the name Zackie Oh.

He was killed at the center of Athens. The trial ended on 3 May 2022, with two men having been found guilty. Zak Kostopoulos was killed on September 21, 2018, from a fatal injury in an incident that took place in the area of Omonia Square in Athens.