Friedrich Wilhelm III, Duke of Saxe-Altenburg

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

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

The death of his older brother Christian (1663) make him the new heir of the duchy of Saxe-Altenburg. When his father died in 1669, Frederick Wilhelm III succeded him; but, because he was only 12 years old, his maternal uncles, the Elector Johann Georg II of Saxony and the duke Maurice of Saxe-Zeitz asumed 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 become extint.

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