Theodor von Sitten was a 4th-century Christian bishop and missionary who served as the first known bishop of Octodurum, which is located in modern-day Martigny, Switzerland. Historical records indicate he attended the Council of Aquileia in 381 and signed a letter from the Synod of Milan to Pope Siricius around 390. While some traditions suggest he might be one of three different bishops named Theodulus, he is widely recognized as a significant figure in the early church of the Valais region.