Theodore I Komnenos Laskaris or Lascaris (Greek: Θεόδωρος Κομνηνὸς Λάσκαρις, romanized: Theodōros Komnēnós Láskaris; c. 1175 – November 1221) was the first emperor of Nicaea—a successor state of the Byzantine Empire—from 1205 to his death. Although he was born to an obscure aristocratic family, his mother was related to the imperial Komnenos clan. He married Anna, a younger daughter of Emperor Alexios III Angelos in 1200. He received the title of despot before 1203, demonstrating his right to succeed his father-in-law on the throne.

The Fourth Crusade forced Alexios III to flee from Constantinople in 1203. Theodore was imprisoned by the crusaders (commonly referred to as "Latins" by the Byzantines), but he escaped. After crossing the Bosporus into Asia Minor (in present-day Turkey), he started to organise the local Greeks' resistance against the Latins in Bithynia in his father-in-law's name. He concluded an alliance with the Seljuq sultan of Rum, but he could not stop the Latins' expansion. Neither could he prevent a claimant to the imperial throne, Alexios Komnenos, from establishing a Byzantine successor state, the Empire of Trebizond, in northern Asia Minor. Theodore's position consolidated only after Tzar Kaloyan of Bulgaria inflicted a crushing defeat on the Latins in the Battle of Adrianople (in Thrace) in 1205.

Greeks fleeing from the Latin Empire — the crusader state that emerged in the Byzantine core territories — swarmed to Asia Minor to live under Theodore's rule. The Latins captured Theodore's father-in-law and made an alliance with Alexios I of Trebizond, but Theodore routed their united troops. He secured the support of most Bithynian aristocrats and seized the domains of those who resisted him. In 1205, he assumed the traditional titles of the Byzantine emperors. Three years later, he convoked a Church council to elect a new Orthodox patriarch of Constantinople. The new patriarch crowned Theodore emperor and established his seat at Theodore's capital, Nicaea. The Orthodox population of the Latin Empire regarded Theodore as the main defender of their Church, but the rulers of Epirus—a realm that developed in the western regions of the Byzantine Empire—debated the legality of his coronation.

Theodore's father-in-law did not abandon his claim to the throne. After being released from prison, Alexios III persuaded the Seljuqs to invade Nicaea, but Theodore defeated them in 1211. The Latin Emperor Henry of Flanders captured important fortresses in western Asia Minor in 1212, but he ran out of soldiers and could not place garrisons in them. Henry implicitly acknowledged the existence of the Empire of Nicaea in his peace treaty with Theodore. Theodore conquered western Paphlagonia on the Black Sea coast from Alexios I of Trebizond. Around 1220, Theodore tried to persuade the Latins of Constantinople to acknowledge his rule, but they refused. Theodore established a powerful state, located in the vicinity of Constantinople, which enabled his successors to expel the Latins from the city and revive the Byzantine Empire in 1261.

Theodore Komnenos Laskaris was born to a noble, but not particularly renowned, Byzantine family around 1175. His parents' names are unknown. If Theodore followed the Byzantine custom of giving his father's name to his firstborn son, his father was called Nicholas. Theodore's mother belonged to an unidentified branch of the imperial Komnenos family and he proudly adopted her surname. Theodore had no fewer than six brothers — Constantine, George, Alexios, Isaac, Manuel and Michael. Manuel and Michael must have been born to a different mother, as they bore the surname Tzamantouros instead of Komnenos. Theodore was also related to the aristocratic Phokas family, most likely through the marriage of one of his aunts.

The Laskaris held estates in western Asia Minor. Both Theodore and his brother, Constantine, had a seal representing Saint George and bearing the inscription Diasorites. The seal expressed their connection to the monastery of Saint George Diasorites, located in Pyrgion in the valley of the river Kaistros.

The contemporaneous historian Niketas Choniates introduced Theodore as a "daring youth and fierce warrior" in his chronicle. The slightly later historian George Akropolites recorded that Theodore was "small in body but not excessively so, quite dark, and had a flowing beard forked at the end". Theodore rose to prominence through his family ties with the Komnenoi. His first extant seal mentions his titles of sebastos and protovestiarites. The first one was a court title, originally reserved for the Byzantine emperors' relatives, but Emperor Alexios III Angelos (r. 1195–1203) started to sell it to wealthy merchants. As protovestiarites, Theodore was the commander of a cadet unit of the guards of the imperial palace.

Emperor Alexios III, who had no sons, wanted to solve the problem of succession by marrying off his two eldest daughters. Late in 1200, he gave his firstborn daughter, Irene, in marriage to Alexios Palaiologos, and her younger sister, Anna, to Theodore. Palaiologos was elevated to the rank of despot, demonstrating his right to succeed his father-in-law on the throne. When Palaiologos died before 1203, Theodore received the same title.