Former Sydney FC young gun Chris Naumoff has retired at just 21 due to a heart condition.

Naumoff was released by the Sky Blues at the end of last season and looked set to sign a three-year deal with Spanish Segunda Division club, CD Numancia.

However, when undergoing a mandatory medical prior to joining the club, it was discovered Naumoff has an abnormality of his heart, known as hypertrophic cardiomyopathy.

It is a condition that puts athletes at risk of cardiac arrest.

“I intend to stay involved with the game of football in one way or another, and hope my example can change the lives of athletes and set a precedent to better testing and awareness for the future and the wellbeing of all athletes,” Naumoff said in a statement.

“How long this condition has affected me is unknown, and how this condition was not discovered during my time as a professional in Australia is a question for another day.”

The 21-year-old made 44 appearances for Sydney FC since joining the club in 2013.