Duke of Holstein-Gottorp
Peter was born in Kiel, in the duchy of Holstein-Gottorp. His parents were Charles Frederick, Duke of Holstein-Gottorp (a nephew of Charles XII of Sweden), and Anna Petrovna (a daughter of Emperor Peter I and Empress Catherine I of Russia). His mother died shortly after his birth. In 1739, Peter's father died, and he became Duke of Holstein-Gottorp as Charles Peter Ulrich () at the age of 11.