The sentence against Alaa Abdel-Fattah, 33, is by far the toughest against any of the liberal, pro-democracy activists behind the 18-day uprising that toppled Hosni Mubarak's 29-year regime.
It is also the first conviction of a prominent activist since former army chief Abdel-Fattah el-Sissi took office as president on Sunday.
In the 11 months since Mr Sissi ousted the country's first freely elected president, Mohammed Morsi, authorities have launched a massive crackdown on Islamists, detaining at least 16,000 and killing hundreds.
That crackdown has overshadowed another campaign against secular activists opposed to what they see as the return of Mubarak-era policies.
Security officials said while Abdel-Fattah was convicted and sentenced in absentia, he did turn up at the Cairo courtroom later and was detained. The absentia sentencing means he faces an automatic retrial, although the conviction stands in the meantime.
The case against him dates to late last year when he was accused of taking part in an "unauthorised" demonstration against a law that places rigid restrictions on street protests. The law gives the Interior Ministry the right to ban any meeting of more than 10 people in a public place.