The court ordered that Mr Ngudjolo be released immediately
Former Congolese militia leader Mathieu Ngudjolo Chui has been acquitted by the International Criminal Court of war crimes and crimes against humanity.
The case related to the 2003 killings of 200 residents of Bogoro village in the mineral-rich Ituri province of the Democratic Republic of Congo.
The court in The Hague heard reports of victims being burned alive, babies smashed against walls and women raped.
Mr Ngudjolo denied ordering the attack, saying he learned of it days later.
Prosecutors said he had enlisted child soldiers to carry out the killings.
But presiding Judge Bruno Cotte said the court acquitted Mr Ngudjolo of all charges after prosecutors failed to prove him guilty of the mass attack.
He ordered his immediate release.
The conflict in Ituri was part of a war that raged in DR Congo following the 1994 genocide in neighbouring Rwanda and involved troops and fighters from several neighbouring countries.