Theodoros I Komnenos Laskaris (Theodore I Comnenus Lascaris) (Greek: Θεόδωρος Α' Λάσκαρις) was the first Emperor of Nicaea (reigned 1204–1221 or 1205–1222).
Theodore Laskaris was born to the Laskaris, a noble but not particularly renowned Byzantine family of Constantinople. He was the son of Manuel Laskaris (b. c. 1140) and wife Ioanna Karatzaina (b. c. 1148). In 1198/9, Theodore married Anna Angelina, daughter of the Byzantine Emperor Alexios III Angelos and Euphrosyne Doukaina Kamatera; she was the widow of her cousin the sebastokratōr Isaac Komnenos.
Theodore distinguished himself during the sieges of Constantinople by the Latins of the Fourth Crusade (1203–1204). He remained in Constantinople until the Latins actually penetrated into the city, at which point he fled across Bosphorus together with his wife. At about the same time his brother Constantine Laskaris was unsuccessfully proclaimed emperor by some of the defenders of Constantinople. In Bithynia Theodore established himself in Nicaea, which became the chief rallying-point for his countrymen.
At first Theodore did not claim the imperial title, perhaps because his father-in-law and his brother were both still living, perhaps because of the imminent Latin invasion, or perhaps because there was no Patriarch of Constantinople to crown him Emperor. In addition, his own control over the Anatolian domains of the Byzantine Empire was challenged, by David Komnenos in Paphlagonia and Manuel Maurozomes in Phrygia. It was only after defeating the latter two in 1205 that he was proclaimed Emperor and invited Patriarch John X Kamateros to Nicaea. But John died in 1206 before crowning Theodore. Theodore appointed Michael IV as the new Patriarch and was crowned by him in March 1208.
At the end of his reign he ruled over a territory roughly coterminous with the old Roman provinces of Asia and Bithynia. Though there is no proof of higher qualities of statesmanship in him, by his courage and military skill he enabled the Byzantine nation not merely to survive, but ultimately to beat back the Latin invasion.
Theodore died in November 1221 and was succeeded by his son-in-law John III Doukas Vatatzes.