The Russian president Vladimir Putin and the heads of the Crimean legislature and cabinet signed a treaty integrating Crimea into the Russian Federation.
Putin said Tuesday that Crimea could only belong to Russia in an address to the Russian parliament.
"Crimea is a legacy. It should belong to a stable country like Russia," he said.
The speech comes 24 hours after 96 percent of Crimean citizens voted to secede from Ukraine and join Russia amid political turbulence in Ukraine.
"These are telling figures," Putin said, to a standing ovation.
"Crimea has always been an integral part of Russia in the hearts and minds of people," he added.
Putin empathised with Russian-speaking Crimeans whose rights, according to him, have been slighted throughout history. Ukrainian leaders isolated the minority and banned them from using their own language, the Russian leader claimed.
The new Crimea will be home to all ethnic groups and will have three languages: Russian, Ukrainian, and Tartar, Putin said.
The Russian president also said that Moscow had not broken international law in its dealings with Ukraine in recent months.
The presence of Russian troops in Crimea was limited to a contingent of 25,000 troops, he said, praising Russia's "shot-free" intervention.
International law does not prohibit a unilateral declaration of independence, Putin said.