The latest whales kept in a facility known as "whale prison" in the Far East of Russia were released on Sunday, the institute responsible for the operation said.
The fate of whales, which were caught for breeding in aquariums, sparked international outrage earlier this year after media photos of them struggled to swim in ice-enclosed waters in tight enclosures.
Moscow eventually bent under pressure to release 93 beluga whales and 11 orcas, which were held for over a year in the secret facilities of Srednyaya Bay near the eastern city of Nakhodka.
Since then, the whales have been released in batches, with the last of the orcas released in August.
The Russian-wide Institute for Fisheries and Oceanography said the operation to release the latest belugas began five days ago.
"In the Primorsky Krai region, the operation to release marine mammals in their natural habit has been completed," the statement said on Sunday.
Russian NGO Sakhalin Watch, which campaigned for the release of whales, said the final 21 belugas were released from two ships on Sunday.
The organization said it was delighted that "the release of all beluga whales occurred and that the" whale prison "finally freed its last prisoners!"
The operation began around 9 am local time and continued until 5 pm, Sakhalin Watch said.
Many scientists and activists have criticized the Oceanography Institute for keeping the details of the release secret, taking no observers on the trip and releasing only a small group of animals at a time rather than all together, which would increase their chances of survival.
Sakhalin Watch said the coastguard forced its members to leave the bay where the whales were being released.
Russia is the only country that catches killer whales and wild belugas for sale in aquariums, a controversial practice made possible by legal loopholes.
Many of the whales kept at Srednyaya Bay facilities should be sent to aquariums in China.
A Change.org pet for the release of whales collected more than 1.5 million signatures, including Hollywood actor Leonardo DiCaprio.