Soheib Debaghi was sentenced in Algiers to one year in prison on charges of encouraging an illegal gathering, insulting an official body and publishing potentially damaging material, the National Committee for the Release of Detainees (CNLD) announced on its Facebook page.
Human rights group Amnesty International said two other activists, Larbi Tahar and Boussif Mohamed Boudiaf, were handed 18-month prison sentences by a court in the western town of El Bayadh, also for posts on Facebook.
The prosecution had called for three-year jail terms, at the hearing held by videoconference, the CNLD said.
Their lawyer, Abdelghani Badi, said on Facebook that Tahar was accused of having insulted President Abdelmadjid Tebboune by calling him "illegitimate", while Boudiaf had criticised the "injustice" of Algeria's judicial system.
Weekly anti-government protests rocked Algeria for more than a year and only came to a halt due to the novel coronavirus outbreak, with the authorities banning marches -- although the opposition had already suspended its gatherings.
A 25-year-old activist, Walid Kechida, has been held since April 27 for having posted memes on social media deemed mocking of Algerian authorities and religion. He risks facing a five-year jail sentence.
Activists and analysts fear Algerian authorities are using the pandemic as a pretext for crushing the "Hirak" protest movement.
According to the CNLD, around 50 people are currently detained over links to the movement.
Late last month, Amnesty urged the authorities to end "arbitrary prosecutions aimed at silencing Hirak activists and journalists" during the pandemic, and called for the release of detainees.
The rights group said that authorities were endangering detainees' health, "given the risks of a COVID-19 outbreak in prisons and places of detention".