In an unusually rapid prosecution, Talaat Sadat, 52, was convicted on Tuesday of defaming the Egyptian armed forces for saying in a television interview earlier this month that unnamed generals had masterminded his uncle's assassination in 1981.
Within minutes of the sentencing, Sadat's supporters shouted outside the court: "This is injustice!" "This is unlawful!"
Sadat is the second prominent political opponent of the government to be sentenced to prison within 12 months. Last December, Ayman Nour, the leading challenger in last year's presidential elections, was sentenced to five years' imprisonment for forgery after a trial that was internationally regarded as failing to meet standards of due process.
An outspoken legislator and a lawyer, Talaat Sadat had accused the government of prosecuting him for political reasons. The US State Department had criticised his prosecution, saying it was concerned that it violated freedom of expression.
Sadat was taken into custody immediately after the verdict, said his aide, Mohsen Eid.
President Sadat was assassinated
on October 6 1981
Egyptian newspapers have been instructed not to report his trial.
There is no appeal against military court verdicts. Sadat's only option is to appeal to Hosni Mubarak, the president.
Sadat had pleaded innocent to charges of "spreading false rumours and insulting the armed forces".
In an interview broadcast on October 4, Sadat said there had been an international conspiracy to assassinate his uncle, and the conspirators included some of Anwar Sadat's personal guards, Egyptian generals, as well as the US and Israel.
"No one from the special personal protection group of the late president fired a single shot during the killing, and not one of them has been put on trial," Sadat told the Saudi TV channel Orbit.
The day after the broadcast, Sadat was stripped of his parliamentary immunity and his trial began on October 11. The parliament will be instructed to drop Sadat's membership, Egypt's official news agency reported later on Tuesday.
President Sadat was shot dead by hardliners in the Egyptian army during a military parade in Cairo on October 6, 1981. The soldiers were opposed to Sadat's landmark peace treaty with Israel of 1979.