The Opsician Theme or simply Opsikion was a Byzantine theme located in northwestern Asia Minor. Created from the imperial retinue army, the Opsikion was the largest and most prestigious of the early themes, being located closest to Constantinople. Involved in several revolts in the 8th century, it was split in three after ca. 750, and lost its former pre-eminence. It survived as a middle-tier theme until after the Fourth Crusade.

The themes or thémata were the main military and administrative divisions of the middle Byzantine Empire. They were established in the mid-7th century in the aftermath of the Slavic migrations to Southeastern Europe and Muslim conquests of parts of Byzantine territory, and replaced the earlier provincial system established by Diocletian and Constantine the Great. In their origin, the first themes were created from the areas of encampment of the field armies of the East Roman army, and their names corresponded to the military units that had existed in those areas. The theme system reached its apogee in the 9th and 10th centuries, as older themes were split up and the conquest of territory resulted in the creation of new ones. The original theme system underwent significant changes in the 11th and 12th centuries, but the term remained in use as a provincial and financial circumscription until the very end of the Empire.