The 30-year-old announced he would leave the club after St. Pauli’s relegation from the Bundesliga was confirmed with a 3-1 defeat to Wolfsburg last weekend.
Vasilj is now available on a free transfer because his contract does not remain valid in the 2. Bundesliga.
Bosnian outlet SportSport claims Mainz are considering a move after Daniel Batz left the club to join Borussia Monchengladbach.
The Bundesliga side are searching for additional depth in the goalkeeping department and view Vasilj as an experienced option with top-flight experience in Germany.
However, Mainz are not alone in the race.
Premier League clubs Brentford and Brighton & Hove Albion are also reportedly monitoring the Bosnia international ahead of the summer window.
Vasilj earned praise throughout a difficult campaign for St. Pauli despite the club’s relegation struggles.
The goalkeeper established himself as one of the side’s more consistent performers and attracted interest with his shot-stopping and composure under pressure.
His availability without a transfer fee has only increased attention around his situation.
Mainz are expected to move carefully in the market this summer as they balance squad depth with financial discipline.
But losing Batz has created an immediate opening that Vasilj could potentially fill.
After confirming his farewell to St. Pauli, the goalkeeper now appears likely to remain in top-level football despite the club’s drop into the second division.