Career
Kostov started to play football in the local side of his home village Kolarovo, Petrich Municipality. In 2007, when he was 12 years old, he joined Pirin 2001 Football Academy, where he honed his footballing skills. In January 2014, Kostov signed with B Group club Pirin Razlog.
Kostov ended the season with 5 clean sheets out of 7 appearances.
In June 2014, Kostov joined Pirin Blagoevgrad. On 9 July 2015, Kostov signed his first professional contract in his career.
He made his A Group debut in a 1–1 home draw against Cherno More Varna on 19 July 2015.