Crimea returned to be a Russian federal province, after a referendum held on March 16, 2014, in Crimea and Sevastopol, and both regions have become within the Russian Federation, as of March 18, 2014.
Crimea was one of the Russian provinces up until the beginning of the fifties of the twentieth century when the authorities of the Union of Soviet Republics decided to transfer their subordination to the Republic of Ukraine.
Disproof
Recurring pro-Kremlin disinformation narrative about the illegal annexation of Crimea, claiming that Crimea voted to rejoin Russia through a legal referendum and painting the idea that Crimea has never belonged to Ukraine.
For historical reference, Crimea was annexed by Catherine II (the Great), then containing Ukraine in 1783, already in violation of a treaty that guaranteed the independence of the Tatar khanate.