Four Moscow-controlled regions of Ukraine will be incorporated into the country later.
Russian officials say the people of the Luhansk, Donetsk, Zaporizhzhia and Kherson regions voted to join the Russia Federation following referendums.
President Vladimir Putin will attend a ceremony at the Kremlin, where the provinces will officially become part of the nation.
Ukraine and the West have dismissed the votes, calling them a "sham."
President Joe Biden says the US refuses to recognise Russia's attempt to annex territory in Ukraine.
Meanwhile, Ukrainian officials say 23 people have been killed in a Russian air strike in the city of Zaporizhzhia.
28 others were injured in a humanitarian convoy.