| Konstantin Tih | |
|---|---|
 Portrait of Konstantin Asen of the frescoes in the Boyana Church | |
| Tsar of Bulgaria | |
| Reign | 1257–1277 |
| Predecessor | Mitso Asen |
| Successor | Ivaylo |
| Died | 1277 |
| Spouse | Irene Doukaina Laskarina Maria Palaiologina Kantakouzena |
| Issue | Michael |
| House | Asen dynasty |
| Father | Tihomir (Tih) |
Konstantin Tih (Bulgarian: Константин Тих Асен) or Constantine I Tikh (Константин I), 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 state—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.