Principality of Brunswick-Wolfenbüttel
He was born in Hitzacker, then the residence of his father Duke Augustus the Younger of Brunswick-Lüneburg (1579–1666) and his second wife Princess Dorothea of Anhalt-Zerbst (1607–1634). The next year his father, at the age of 55, assumed the rule in the Principality of Brunswick-Wolfenbüttel after his Welf cousin Duke Frederick Ulrich had died childless.