*Club domestic league appearances and goals, correct as of 21:14, 1 October 2023 (UTC) ‡ National team caps and goals, correct as of 21:14, 1 October 2023 (UTC)
Born in Sofia, Kostov started his career in the local Septemvri Sofia.[3] On 7 September 2020 he completed a move to the Benfica academy.[4] In June 2023 it was announced that Kostov won't renew his contract with Benfica and will leave the team.[5] Few days later he returned in Bulgaria and signed a 3-year contract with Lokomotiv Plovdiv.[6] He scored his debut goal on 7 September 2023 in a league match against Hebar Pazardzhik, but minutes later was injured and set to miss around 6 months.[7]