Jean-Paul Akayesu, a Hutu, was convicted of "meticulously organised" genocide, crimes against humanity, murder, torture and rape.
His lawyer said that Akayesu, who maintained his innocence throughout his 18-month trial, would appeal.
Pierre Prosper, the prosecutor, said the case provided "a road map for how we are to proceed" in genocide prosecutions.
He said he was looking forward to going to Akayesu's home town, the central Rwandan village of Taba, and telling survivors of the slaughter: "We did it. We achieved justice."
But in Rwanda, where the slow pace of the tribunal's work has been criticised, the verdict was greeted dispiritedly. In nearly four years, this is the first conviction. Patrick Mazimhaka, a state minister, said: "It has gone on for so long and I think people have given up."
At yesterday's session the chief judge, Laity Kama of Senegal ordered Akayesu to stand as the verdicts were read. Each time a guilty verdict was read out - on nine of fifteen counts - Akayesu winced.
The International Criminal Tribunal for Rwanda said the judgment against Akayesu was also the first, by an international court, for sexual violence.
Akayesu, a former teacher born in 1953, was convicted of ordering the killings of 2,000 people who sought his protection in Taba. They were among the 800,000 Rwandans, mostly minority Tutsis but also moderate Hutus, who were butchered with machetes and nail-studded clubs in a genocide orchestrated by extremists in the former Hutu government.
During yesterday's tribunal session, Laity Kama said the three-judge panel had rejected arguments that Akayesu was helpless to prevent the killings. A mayor "had a lot of power", the judge said.
Akayesu was held over in Arusha, Tanzania, pending a pre-sentencing hearing on 28 September. The maximum penalty is life imprisonment.
"The penalty doesn't match the crime," said Chantal Kayitesi, who heads a group of widows of the Rwandan slaughter. "But we have to recognise the difference between international laws and ours."
The tribunal is holding 31 people in Arusha, four of whom are on trial. Eight others have been indicted, but not apprehended, and an elderly Rwandan minister is in custody in Texas.Reuse content