According to the Wikipedia page (lol):

Treaty on the Creation of the USSR

On 28 December 1922, a conference of plenipotentiary delegations from the Russian SFSR, the Transcaucasian SFSR, the Ukrainian SSR and the Byelorussian SSR approved the Treaty on the Creation of the USSR[27] and the Declaration of the Creation of the USSR, forming the Union of Soviet Socialist Republics.[28] These two documents were confirmed by the first Congress of Soviets of the USSR and signed by the heads of the delegations,[29] Mikhail Kalinin, Mikhail Tskhakaya, Mikhail Frunze, Grigory Petrovsky, and Alexander Chervyakov,[30] on 30 December 1922.

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