Navalny while she is being tried for alleged "extremism", her associates said Monday.
Ksenia Fadeyeva, who led Navalny's now-banned Anti-Corruption Foundation group in the Siberian city of Tomsk, faces up to 12 years in prison if convicted.
Fadeyeva had been under house arrest.
She was added to Russia's list of «terrorists» in January 2022 and went on trial in August.
Moscow has used laws on «terrorist» and «extremist» bodies to hand out years-long jail sentences to critics, including Navalny and his top allies.
«Ksenia Fadeyeva was sent to jail,» Navalny's team wrote on social media.
A judge in the case sided with the prosecution which argued house arrest was too lenient, Andrei Fateyev, a colleague of Fadeyeva said Monday.
Fadeyeva was elected an opposition member of the Tomsk city parliament in 2020 — weeks after Navalny was poisoned with what doctors said was the Novichok nerve agent while on a campaign trip to the city.
Navalny himself is currently serving 19 years in prison, also on «extremism» charges.
Russia outlawed Navalny's organisation in 2021 and has escalated a ruthless legal crackdown on those of his associates who refused to flee the country.
That campaign has intensified after Moscow launched its assault on Ukraine last February.
The Memorial human rights group — which Russia has also liquidated — said Monday Fadeyeva was a political prisoner.
Navalny's team said Fadeyeva had led investigations into corruption in the region, shone a light on societal problems and «engaged in legal political activities.»
«Ksenia should be free,» it said in a post on social media.