The rights activists include Yara Sallam and Sanaa Seif, described by Amnesty International as "prisoners of conscience."
The Cairo court also ordered each of the 23 defendants to pay a fine of 10,000 Egyptian pounds (about USD 1,390).
Some of the activists had supported the military's overthrow of Islamist president Mohamed Morsi in July last year which unleashed a deadly crackdown on his Islamist supporters.
They had since turned on the new authorities as it extended a crackdown on all protests.
The defendants were accused of holding an illegal protest on June 21 calling for the release of detainees and the annulment of a law that bans all but police-sanctioned demonstrations.
"The ruling is political, it has no legal grounding," alleged Ahmed Ezzat, one of the defence lawyers, after judge Abdelrahman al-Zawary pronounced his verdict.