Friedrich Wilhelm III, Duke of Saxe Altenburg

Friedrich Wilhelm III (b. Altenburg, 12 July 1657 – d. Altenburg, 14 April 1672), was a duke of Saxe-Altenburg.

He was the second son of Friedrich Wilhelm II, Duke of Saxe-Altenburg and Magdalene Sybille of Saxony, his second wife.

The death of his older brother Christian (1663) made him the new heir of the duchy of Saxe-Altenburg. When his father died in 1669, Friedrich Wilhelm III succeeded him; but, because he was only twelve years old, his maternal uncles, the Elector Johann Georg II of Saxony and the duke Maurice of Saxe-Zeitz, assumed the guardianship of the new duke and the regency of the duchy.

Only three years later, the young duke died of smallpox. With his death, the branch of Saxe-Altenburg became extinct.

The duchy of Saxe-Altenburg was split between the branches of Saxe-Gotha and Saxe-Weimar; but, on the basis of the will of the duke Johann Philipp of Saxe-Altenburg (Friedrich Wilhelm III's uncle) the greater part of the duchy was retained by Saxe-Gotha, because they were the descendants of Elisabeth Sophie, the only daughter of Johann Philipp, who declared her the general heiress of the family in case of the extinction of the male line.