Demanding his release, the United States said the conviction calls into question Egypt's commitment to democracy and the rule of law.
Ayman Nour, who has consistently denied the charges and is on hunger strike, finished a distant second to President Hosni Mubarak in September's elections, Egypt's first contested presidential race.
He lost his parliamentary seat in November's legislative polls.
The guilty verdict provoked uproar from the hundreds of Nour's supporters in and around the courtroom.
Nour's wife, Gameela Ismail, shouted "Down to Mubarak".
Announcing that Nour would appeal, his lawyers said it was "a political decision, not a judicial sentence."
Nour appeared pale when he arrived in the court.
He had been in custody for a week after starting a hunger strike to protest his detention on December 5.
Nour had pleaded not guilty to charges that he ordered forged signatures to register his Al-Ghad party last year.
During the trial, one of his six co-defendants retracted his confession, telling the court that security officials had coerced him to make a statement accusing Nour of forgery.