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. In particular, Kostopoulos was found trapped inside a jewelry store on Gladstonos Street under unspecified circumstances. He was beaten by the shop owner and a bystander broker, mistaking him for a robber. Following the beating, the bloodied Kostopoulos was arrested by at least 8 police officers who rushed to the scene of the violent incident. Kostopoulos expired while being transported by ambulance to the hospital.

After his death, protest marches and gatherings were held in various cities of Greece, while associations, parties and other organizations issued statements, mainly condemning the phenomenon of self-righteousness, which occupied the Greek public opinion for a number of months. Annual marches and commemorative events are held on September 21 in Athens, Thessaloniki and other cities.