Since Peter III. had allegedly behaved with inappropriate silliness even during his wedding festivities with Catherine, this angered both, his young wife and large sections of the Russian people. Catherine urged her husband to "exercise moderation", even and especially in politics, but Peter III.'s first acts of state just brought disadvantages to Russia.

Because of this, Catherine and her confidants then planned a risky coup d'état. She first secured the support of several guard regiments, in which the Orlov brothers served, among others, and then had herself proclaimed empress on July 9th, 1762, while Emperor Peter III. was declared deposed. Catherine advanced with the Guard to Peterhof, where her husband was staying at that time. Peter initially fled to Kronstadt, but returned and later signed his abdication document in Oranienbaum. Catherine was declared the sole ruler of Russia by Metropolitan Sechin in the Kazan Cathedral of Saint Petersburg on the same day. Peter III. was taken prisoner and died under "unexplained" circumstances on July 17th, 1762.