Haradinaj faces a maximum life sentence if convicted of war crimes and crimes against humanity including murder, rape, and torture against Serbs.
Haradinaj was a commander of the separatist Kosovo Liberation Army in the 1990s.
Two other alleged rebel fighters are also standing trial on similar charges. All three have pleaded not guilty.
Kosovo Prime Minister Agim Ceku said Haradinaj would not only be defending himself, but Kosovo's "war for freedom."
The trial comes as Belgrade is rejecting a UN plan to grant Kosovo limited independence.
Billboards with the slogan "With Ramush" have sprouted across the province in recent days, and hundreds of supporters cheered as he left Kosovo to return to The Hague for his trial.
After the war, Haradinaj briefly served as prime minister of UN-administered Kosovo. He resigned in March 2005 when he was indicted by the UN Tribunal.
He was allowed to return home while he awaited trial and resumed limited political responsibilities as leader of the Alliance for the Future of Kosovo party.