John Sigismund was the third Duke of Prussia, reigning in that capacity for a very short time in the 16th Century.

He was born on Nov. 8, 1572, in Halle. His father was Joachim Frederick, the Elector of Brandenburg, and his mother was Catherine of Brandenburg-Küstrin. Son succeeded father as Elector in 1608. By that time, he had begun to serve as regent for the Duke of Prussia, Albert Frederick, taking over for his father in that regard. The duke had suffered from mental health issues and had three successive regents running the government for him. John Sigismund turned out to be the last, succeeding Albert Frederick on Aug. 27, 1618.

John Sigismund had married Anna of Prussia, one of the daughters of Albert Frederick, in 1594. They had eight children, five of whom survived into adulthood: George William (1595), Anne Sophia (1598), Maria Eleonora (1599), Catherine (1602), and Joachim Sigismund (1603).

John Sigismund embraced the Lutheran faith, as did many of his subjects. However, he converted to Calvinism in 1613. Attempts to convince his subjects to do the same largely failed.

He had a stroke from which he did not recover, dying on Dec. 23, 1619. His oldest son, George William, succeeded him.