Seif, whose older brother and dissident Alaa Abdel Fattah has also been imprisoned since February 2015, was convicted of "insulting the judiciary," a judicial official said.
She had been caught up in a renewed crackdown on secular dissidents who had called for protests this month against President Abdel Fattah al-Sisi for handing over two islands to Saudi Arabia.
Dozens were arrested in the lead-up to an April 25 protest in Cairo which ended with police dispersing the rally using tear gas and detaining more activists.
Seif's conviction stems from a prosecution summons on suspicion of inciting protests.
She did not attend and was charged with "insulting a member of the prosecution," the judicial official said.
The verdict may be appealed, although Seif has said she will not contest it.
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In a Facebook post, Seif said she did not attend Wednesday's hearing but was going to hand herself in.
"Simply, I do not have the energy to deal with their measures," she wrote.
Seif had been pardoned last October after she was imprisoned over a 2014 protest outside the presidential palace in Cairo.
Sisi, a former army chief who was elected president after overthrowing his Islamist predecessor in 2013, has been accused by activists of installing a heavy-handed regime that tolerates no dissent.
After president Mohamed Morsi's overthrow, a police crackdown killed hundreds of Islamist protesters, while a jihadist insurgency has killed hundreds of policemen and soldiers.
The crackdown has extended to secular and liberal dissidents over the past two years.
Earlier this month, Sisi handed over two islands in the Straits of Tiran to Saudi Arabia during visit by King Salman to Cairo.
The government says the islands had always belonged to Saudi Arabia and Egypt merely administered them since the 1950s, but critics accuse Sisi of "selling" them in exchange for Saudi largesse.