Otto III, Count of Weimar-Orlamünde
Grave stone for Counts Otto III and Otto V (d. 1315) in Himmelkron Abbey
Born	1244
Died	June 1285
Buried	Himmelkron Abbey in Himmelkron
Noble family	House of Ascania
Spouse(s)	Agnes of Truhendingen
Father	Herman II, Count of Weimar-Orlamünde
Mother	Beatrix of Andechs-Merania

Otto III. Weimar-Orlamünde, sometimes called Otto IV (1244 – June 1285) was a German nobleman. He was a member of the House of Ascania and a titular Count of Weimar-Orlamünde. He was the ruling Count of Weimar and Lord of Rudolstadt and Plassenburg.

He was the son of Count Herman II and his wife, Beatrix of Andechs-Merania. In 1248, Otto III and his elder brother Herman III inherited the Franconian possessions of their maternal uncle Otto II. They divided their possessions, with Otto receiving Weimar, Rudolstadt and Plassenburg and Herman receiving Orlamünde.

On 29 December 1279, Otto III founded Himmelskron Abbey. He died in June 1285, and was buried in the collegiate church of the abbey.

Otto III was married to Agnes of Truhendingen (d. 13 May 1285). Together, they had the following children:

• Otto "the Younger" (d. before September 1318)
• Otto V (d. 1315)
• Herman (d. 1319)
• Agnes (d. 1354) Abbess at Himmelkron monastery

• 
  Sculpture on his grave
• 
  The sarcophagus of count Otto III of Weimar-Orlamünde
• 
  His coat of arms

• Helmuth Meißner: Stiftskirche, ehemaliges Kloster und Schloss Himmelkron, Munichen and Berlin, 1998
• C. Chl. Freiherr von Reitzenstein: Regesten der Grafen von Orlamuende aus Babenberger und Ascanischem Stamm, Historischer Verein für Oberfranken, Bayreuth, 1871
• Theodor Zinck: Himmelkron — Beschreibung seiner Vergangenheit und Gegenwart, Bayreuth, 1925, pp. IV and 4