Russia's President Vladimir Putin on Wednesday approved the ratification of the agreement on the "admission" of Ukraine's Donetsk, Luhansk, Zaporizhzhia, and Kherson regions into the Russian Federation.
He also signed a federal constitutional law on the inclusion of four regions within Russian territory.
On Tuesday, the Russian parliament ratified agreements on annexing the Ukrainian regions.
Putin signed agreements last Friday with separatist authorities of the breakaway Ukrainian regions on joining Russia, following "referendums" held on Sept. 23-27.
The vote came more than seven months into Russia's war on Ukraine that began on Feb. 24.
The "referendums" have been condemned by the international community, with European nations and the US calling them a "sham" and underlining that they would not be recognized.
Ukrainian President Volodymyr Zelenskyy said Putin's decision to annex four regions is invalid and has no legal consequences.