Posch started his senior career at
Admira Wacker Amateure in the
Austrian Regionalliga, before joining the youth ranks of
TSG Hoffenheim in 2015. On 28 September 2017, he made his senior debut for the first team of TSG Hoffenheim, by starting in a 2–1 away defeat against
Ludogorets Razgrad in the
Europa League. A year later, on 19 September 2018, he made his
UEFA Champions League debut in a 2–2 away draw against
Shakhtar Dontesk during the
2018–19 season. He scored his first goal for the club in a 5–0 victory over
FC Köln on 15 October 2021. On 1 September 2022, Posch joined
Serie A club
Bologna on loan with an option and obligation to buy. Two months later, on 6 November, he netted his first goal in a 2–1 victory over
Torino. The transfer to Bologna was made permanent on 31 May 2023. On 3 February 2025, Posch moved on loan to
Atalanta, with an option to buy. On 1 September 2025, Posch joined
Serie A club
Como on loan with an obligation to buy. After the loan ended, Posch joined
Como, and was immediately loaned to
Bundesliga club
Mainz 05 until the end of the season. ==International career==