Melbourne Victory has announced the departure of defender Kasey Bos, who has joined Bundesliga side 1. FSV Mainz 05 for an undisclosed transfer fee.
The 21-year-old defender made his senior debut with Victory during the 2023/24 season and became a valuable contributor after establishing himself as a regular in the senior team during the 2024/25 season.
He then built on his success at national team level, with a maiden Socceroos call-up for the June 2025 FIFA window, earning a place in the squad for Australia’s historic 2-1 win over Saudi Arabia to qualify for the 2026 FIFA World Cup.
Kasey also capped off a breakout year by being named the Rick Smith Young Player of the Year at the 2024/25 Victory Medal Awards Night.
Mainz finished sixth in the Bundesliga this season and have earned a spot in the Europa Conference League play-offs.
“I’ve loved every minute I’ve played at Melbourne Victory,” Boy told Victory’s official website.
“The club has helped shape my development and build the foundations necessary to take this next step overseas.”
Melbourne Victory Director of Football John Didulica added: “For a player to progress from our Academy to the A-League team, and now to a top-six Bundesliga club, is a testament to the belief and hard work of many, not least Kasey and his family.
“We are incredibly proud to have been part of his journey so far, and have no doubt he will continue to achieve great things with 1. FSV Mainz 05.
“Kasey’s progress in such a short time, highlights the work of our coaches and support staff, and we have every confidence that our program will continue to produce young talent, providing our members and fans with the opportunity to support our boys at the start of their careers and then continue to follow them through their successes abroad and as part of the Australian National Teams.”
