25 December 1991 – Russian Soviet Federative Socialist Republic becomes the Russian Federation.
On 25 December 1991 President Mikhail Gorbachev resigned and handed over his powers to President Boris Yeltsin of the Russian Federation. The Soviet flag was lowered from the Kremlin for the last time and replaced with the Russian national flag. The next day the Soviet Union was formally dissolved at a session of the Supreme Soviet’s upper chamber, the Soviet of the Republics.
Source: ‘Dissolution of the Soviet Union,’ Wikipedia