Khan Kormesiy was the third Khan of Danubian Bulgaria ruling from 721 to 738. His name is, however, missing from the "Imennik (List of names) of Bulgarian Khans. The only information given in the Imennik is that he ruled for 28 years and was a descendant of the royal family of Dulo. As Western cronicles quote the name of Kormesiy as "the third ruler of the Bulgarians", it is customarily though that the anonymous ruler mentioned in the "Imennik" was no one else but Kormesiy.

The name of Kormesiy is found in relation to the events surrounding the peace treaty in 716. According to the Byzantine chronist Teophanus the treaty was signed by Kormesiy on behalf of the Bulgarian side. It is, however, firmly proved that Bulgaria was ruled by Khan Tervel at that time. Based on that, some researchers claim that Kormesiy shared rulership with Tervel and signed the treaty as his co-ruler. The name of Kormesiy is also found in the inscriptions of the Madara Rider. The surviving part of the text speaks of an annual gold tribute that Kormesiy received from the Byzantine Emperor - it seems as if the peace treaty was re-established during his rule. The end of the inscription mentions a worsening of Bulgarian relations with the Byzantine Empire.

Kormesiy is not mentioned in any other historical context. The fact that there is no record of wars between Bulgaria and the Byzantine Empire during his reign, however, implies that he sustained the peace between the two countries.