Kira Yarmysh posted on X that a service for Navalny would be held on Friday at 2pm Moscow time (2200 AEDT) in the Church of the Icon of the Mother of God in the Moscow district of Maryino.
She told people to get there early.
Navalny would then be buried at the Borisovskoye cemetery, she said.
Navalny, 47, died at an Arctic penal colony on February 16.
His allies have accused President Vladimir Putin of having him murdered because the Russian leader could allegedly not tolerate the thought of Navalny being freed in a potential prisoner swap.
The Kremlin, which casts Navalny and his supporters as US-backed extremists, has denied state involvement in his death and said it is unaware of any agreement to free Navalny before his death.
Navalny's death certificate, according to supporters, said he died of natural causes.
His mother accused the authorities of trying to blackmail her into holding a private funeral for her son by initially withholding his body, an assertion the Kremlin called absurd.
Navalny's allies had been looking for a hall to accommodate his supporters at a farewell ceremony, but said on Tuesday they had been refused everywhere.
with Reuters
