Waleed Abulkhair said on Wednesday he would remain out of jail pending an appeal of the sentence.
The court in the western city of Jeddah handed down the sentence late Tuesday, citing the lawyer's signing of a petition criticizing harsh sentences given to 16 activists arrested in 2011.
The sentence was criticized by rights group Amnesty International, which described it as "yet another sign of the arbitrary nature of Saudi Arabia's justice system."
Hassiba Hadj Sahraoui, Amnesty's MENA program deputy director, said the trial also illustrated how the authorities "abuse the justice system to silence peaceful dissent in Saudi Arabia".
"This conviction and prison sentence should be quashed. And the pending charges should be dropped," said Sahraoui.
Authorities earlier this month briefly held Abulkhair for setting up an alleged "unauthorized" gathering hall where he held meetings with reformists in the absolute monarchy. He was later freed on bail.
The lawyer is already facing trials in several cases linked to his activism.
In June 2012, he was accused of "disrespecting the judiciary system... contacting foreign organizations and signing a petition demanding the release of detainees," some of whom were being held over alleged terror links, his wife Samar Badawi said at the time.
Three months earlier, authorities had banned him from travelling to the United States where he had been due to attend a forum organized by the State Department.
And in February 2011, he signed two other petitions demanding political reform in the ultra-conservative kingdom, where political parties are banned.
Abulkhair has created a group on Facebook -- Monitor of Human Rights in Saudi -- which has nearly 7,000 members.