Bundesliga
Following an impressive breakout season at boyhood club Melbourne Victory, Kasey Bos has elected to join Mainz ahead of the 2025/26 campaign, swapping the A-League for the German top-flight. bundesliga.com fills you in on the Australian defender...
Data correct as of 13 June 2025
Kasey Bos
Age: 21 (born 8 May 2004)
Club: Mainz
Position: Left-back / Left-winger
Country: Australia
Bos spent part of his youth career at local rivals Melbourne City before ultimately joining Melbourne Victory in 2021. There, he rose through the ranks, playing an important role in the youth side's promotion to the second tier of Australian football in early February 2024, which earned him a first team call-up later that month.
Bos made his full professional debut on 25 February 2024, coming off the bench in a 1-0 defeat to Central Coast Mariners. Despite the defeat, he continued to be selected for the Victory's first team, impressing at every opportunity. This strong end to the season saw the young full-back play in the A-League elimination final; a game in which he grabbed the headlines as he assisted an 88th minute equaliser in a remarkable come-from-behind triumph for the Victory.
Pushing on in the 2024/25 campaign, Bos featured much more regularly, starting in Victory's 2024 Australian cup defeat to Macarthur FC before going on to amass four goals and two assists in 26 appearances in the A-League, as Victory reached a consecutive elimination final, losing this time to rivals City. However, such strong individual performances also earned him a first international call-up to the Australian Socceroos' June 2025 World Cup qualifiers squad, though he has yet to feature.
Set to be a Carnival Club player from July onwards, Bos will become the third active Australian player currently in the Bundesliga, following in the footsteps of the St. Pauli duo of Jackson Irvine and Connor Metcalfe.
Plays a bit like: Hiroki Ito
Did you know?
Kasey's older brother Jordan is also a professional footballer, also plying his trade as a left-back but for Belgian Pro League side Westerlo. The brothers played together as part of Melbourne City's academy, before Kasey moved to rivals Victory.
Additionally, the siblings are in possession of dual Australian and Dutch citizenship, but have both elected to represent their country of birth, with Jordan a 19-time senior Australia international.
What they're saying
“Kasey Bos is a hard-running player on the left flank with strengths in both attack and defence. Additionally, his brave style of play was a strong fit for what we were looking for. He has already been able to prove his talents in the A-League.” - Mainz sporting director Niko Bungert
"The Bundesliga is an exciting league and the level is high. Mitchell Langerak, one of my teammates in Melbourne, was able to give me several tips already. I’m now looking forward to getting to know the team and the fans, in addition to exploring more of Mainz." - Bos on joining Mainz
"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 Mainz." - Melbourne Victory director of football John Didulica