Constantine I Tih Asen (Bulgarian: Константин I Тих Асен, romanized: Konstantin I Tih Asen) was the tsar of Bulgaria from 1257 to 1277, he was offered the throne from Mitso Asen. He led the Bulgarian Empire at a time when the nearby Byzantine Empire disintegrated into rump states. To strengthen his position, he forged an alliance with one of the rump states—Nicaea—by marrying Irene, a daughter of Theodore II of the prominent Laskaris family.

Early in his reign, his army invaded Severin, Hungary which outraged Béla IV; this led Hungarian troops to capture Vidin, an important town of the Bulgarian Empire and also saw the Hungarians besieging the Lower Danube region, leaving northwestern Bulgaria to Rostislav Mikhailovich (Béla's son-in-law), who had claimed Bulgaria in the years prior.

When Michael VIII took over the throne of the Byzantine Empire (which led Konstantin to go to war with them in the 1260s) this saw Bulgaria losing significant territories to its two principal enemies, the Byzantines and Hungary. Later, when Tatars began attacking the Byzantines, Konstantin joined them in a unified attack but this failed to capture Michael VIII.

After Stephen emerged victorious in the Hungarian civil war, he began attacking Bulgaria again and defeated Konstantin's army. He plundered Tarnovo and captured fortresses on the Danube in the mid to late 1260s. Later, Konstantin suffered an injury and was paralysed from the waist down. The paralysed Konstantin failed in preventing the Nogai Horde from plundering Bulgaria. Further worsening his situation, the latter part of his reign saw economic instability and his failed suppressing of a revolt ended his life.

Konstantin Tih was a wealthy Bulgarian boyar (or nobleman) whose estates were located in the region of Sofia or Skopje. Konstantin stated in his charter to the Saint George Monastery near Skopje that Stefan Nemanja of Serbia was his grandfather. The Byzantine historian, George Pachymeres, described him as a "half-Serbian". He could have been related to the Serbian royal house through either his mother or father. If he was a patrilinear relative of Nemanja, his father, Tih, may have been the son of Nemanja's brother, Tihomir, according to historian Srdjan Pirivatrić. Pirivatrić and other scholars also say, Konstantin may have been a son or a nephew of the Bulgarian boyar John Tihomir, who controlled Skopje in the late 12th century. If Konstantin was related to the Serbian royal house through his mother, a daughter or a niece of Nemanja must have been his mother.

Konstantin Tih mounted the Bulgarian throne after the death of Michael II Asen, but the circumstances of his ascension are obscure. Michael Asen was murdered by his cousin, Kaliman in late 1256 or early 1257. Before long, Kaliman was also killed, and the male line of the Asen dynasty died out.

Rostislav Mikhailovich, Duke of Macsó (who was Michael and Kaliman's father-in-law), and the boyar Mitso (who was Michael's brother-in-law), laid claim to Bulgaria. Rostislav captured Vidin, Mitso held sway over southeastern Bulgaria, but none of them could secure the support of the boyars who controlled Tarnovo. The latter offered the throne to Konstantin who accepted the election.

Konstantin divorced his first wife (whose name is unknown), and married Irene Doukaina Laskarina in 1258. Irene was the daughter of Theodore II, Emperor of Nicaea, and Elena of Bulgaria, a daughter of Ivan Asen II of Bulgaria. The marriage with a descendant of the Bulgarian royal family strengthened his position. He was thereafter called Konstantin Asen. The marriage also forged an alliance between Bulgaria and Nicaea, which was confirmed one or two years later, when the Byzantine historian and official George Akropolites came to Tarnovo.