Peter August Friedrich, Duke of Schleswig-Holstein-Sonderburg-Beck (7 December 1697 – 22 March 1775) was a Duke of Schleswig-Holstein-Sonderburg-Beck. He was the fifth and youngest son of Frederick Louis, Duke of Schleswig-Holstein-Sonderburg-Beck and his wife Duchess Luise Charlotte of Schleswig-Holstein-Sonderburg-Augustenburg.

Born in Königsberg, Brandenburg-Prussia, Peter August became Governor General of Reval (Tallinn), Governor of Estonia, and a Field Marshal in the Russian Imperial Army. He died in Reval.

He was only the titular duke, because he did not inherit the domain of Beck. It had been inherited by his father's older brother, Duke August (1652–1689), and nephew Frederick William I (1682–1719), and was then sold by the latter's widow to Peter August's older brother, Duke Frederick William II (1687–1749), in 1732. But he re-sold it in 1745 to Baroness Magdalena von Ledebur-Königsbrück. Thus, by the time that Peter August became the head of this cadet branch of the Danish royal dynasty, he was "Duke of Schleswig-Holstein-Sonderburg-Beck" in name only.

He is the male line ancestor of Margrethe II of Denmark, Harald V of Norway and Charles, Prince of Wales.

Peter August married Princess Sophie of Hesse-Philippsthal, the daughter of Philip, Landgrave of Hesse-Philippsthal and his wife Countess Katharina Amalie of Solms-Laubach, at Rinteln on 5 September 1723. Sophie died on 8 May 1728 at age thirty-three. Peter August and Sophie had two sons and one daughter:

...
Wikipedia