In January 2014, Kostadinov was included in Cherno More's 25-man squad for their training camp in Turkey.[4] Martin made his full first team début in a 4-0 away win against Lyubimets on 9 March 2014, playing as left back for 60 minutes. On 27 October 2015, he scored his first goal for the club in a 5–0 away win over Vihar Stroevo in the Bulgarian Cup.[5] Kostadinov was used sparingly in the next two years following his début, but made significant progress during the 2016–17 season when he was named in the starting XI for 12 matches. Martin won the Man of the match award in the final two league fixtures, a 1–3 home defeat by Ludogorets Razgrad on 28 May 2017[6] and a 2–2 away draw against Levski Sofia on 31 May 2017,[7] when he scored his first league goal.[8]