Kostadin Dyakov (; born 22 August 1985) is a Bulgarian former footballer who played as a midfielder.
He was raised in Levski Sofia's youth teams. After that he played as a loaned footballer for Vihren Sandanski and Rodopa Smolyan. He signed than with Chernomorets Burgas in June 2007 on free transfer from Levski Sofia and joined on 8 October 2009 for a trial to FC Schalke 04.
On 14 July 2016, Dyakov signed with Montana. He left the club in June 2017 when his contract expired.
On 5 September 2017, Dyakov signed with Second League club Maritsa Plovdiv.
Dyakov plays as a right or defensive midfielder.