"This is the will of millions of people," he said in a speech before hundreds of dignitaries in the St George's Hall of the Kremlin.
The ceremony took place three days after the completion of hastily staged referendums in which Moscow's proxies in the occupied regions claimed majorities of up to 99% in favour of joining Russia.
Ukraine and Western governments described those votes as bogus, illegitimate and conducted at gunpoint.
In a speech repeatedly interrupted by applause, Putin declared that Russia had four new regions.
He urged Ukraine to cease military action and return to the negotiating table. Kyiv has vowed to recapture all the lands seized by Russia and said that Russia's decision to annex the territories had destroyed any prospect of talks.
By Mark Trevelyan
(Writing by Mark Trevelyan; editing by Jonathan Oatis/Guy Faulconbridge)
Russian President Vladimir Putin chairs a meeting with the members for Russian security Council via a video conference at the Kremlin in Moscow, Russia, 29 September 2022. EPA-EFE/GAVRIIL GRIGOROV/SPUTNIK/KREMLIN / POOL MANDATORY CREDIT