Kislyak on September 27 formally assumed his duties representing Russia's Republic of Mordovia in the upper chamber of the legislature, Russian state media reported, RFE/RL said.
Both chambers of the Russian parliament nearly always rubber-stamp Kremlin-backed initiatives.
Kislyak, 67, served as ambassador to the United States from 2008 until July, when he departed Washington amid concerns about his contacts with senior members of Trump's team during and after the 2016 U.S. presidential campaign.
Read alsoRFE/RL: New Russian ambassador to U.S. says ready to improve relationsThese contacts included interactions with former U.S. national security adviser Michael Flynn, Attorney General Jeff Sessions, and Trump's son-in-law and senior adviser Jared Kushner.
Trump has rejected suggestions of any impropriety in these contacts.
Kislyak was also present at a White House meeting in May at which Trump reportedly shared classified information with Russian Foreign Minister Sergei Lavrov.
Read alsoMcCain: Russian diplomat didn't belong in the Oval Office – The HillTrump defended that meeting, saying he had an "absolute right" to relay "facts pertaining to terrorism and airline flight safety."
Before serving as ambassador to the United States, Kislyak was deputy foreign minister from 2003 until 2008. He previously served as Russian ambassador to Belgium and the head of Russia's mission to NATO.
Kislyak was replaced in Washington by Anatoly Antonov, a veteran diplomat who is under European Union sanctions for his role in Moscow's interference in Ukraine.