Images from Omarska shocked the world during the war
The 34-year-old waiter is named in two indictments and faces 21 counts of crimes against humanity and 25 of war crimes - among them actions at the infamous detention camps of Omarska and Keraterm.
Tribunal spokesman Jim Landale said that although Mr Knezevic was not an official at the camps, he was one of "the so-called camp visitors who came to kill, beat or abuse prisoners".
Infamous camps
Mr Knezevic is expected to appear before the court to enter a plea next week.
The charges against him are punishable by a maximum sentence of life in prison.
Omarska and Keraterm were among the most notorious of 39 detention camps set up at the beginning of the war in Bosnia - part of what prosecutors say was a Serbian plan to ethnically cleanse the area of non-Serbs.
About 6,000 Muslims and Croats were held in Omarska, a former mining complex 20 kilometres (12 miles) from the Bosnian town of Prijedor.
Images of half-starved, semi-naked, prisoners being held there shocked the world and led to international calls for intervention.
According to the UN indictment, some 7,000 non-Serbs were also held in the nearby camp of Keraterm, where some were starved, beaten, sexually assaulted and killed.
Still at large
In recent weeks, five other Serb suspects turned themselves in after Belgrade urged all Serb war crimes suspects to surrender rather than face arrest and extradition.
The five, who are now in custody of the tribunal are:
Another suspect, former Yugoslav army officer Vladimir Kovacevic - who is wanted for the army shelling of the Croatian city of Dubrovnik - has declared he also intends to surrender voluntarily next week.
Seventeen other Serbs indicted by the UN court for alleged war crimes in the former Yugoslavia in the 1990s remain at large.
Among them two of the most wanted suspects: Bosnian Serb wartime leader Radovan Karadzic and his military chief Ratko Mladic.