Nõmme United Hein began his youth career at
JK Loo at the age of eight, before moving to the youth team of
Nõmme United in 2015. He made five senior appearances for Nõmme United in the Estonian third tier,
Esiliiga B, before joining the
academy of English club
Arsenal in May 2018.
Arsenal Hein signed his first professional contract with the club in May 2019. He appeared in the Arsenal squad as an unused substitute for the first time on 29 October 2020, in the
Europa League match against
Dundalk. After playing for the Arsenal first-team in the
2021–22 pre-season, including his non-competitive debut against
Hibernian, Hein signed a new long-term contract with Arsenal on 17 September 2021. On 10 November 2022, Hein made his debut for the Arsenal first-team in the
EFL Cup third round against
Brighton & Hove Albion at the
Emirates Stadium, conceding a foul in the penalty area and the resulting spot-kick against
Danny Welbeck as Arsenal lost 3–1.
Loan to Reading On 24 January 2022,
Reading announced the signing of Hein on loan from
Arsenal until the end of the
2021–22 season.
Loan to Valladolid On 13 August 2024, Hein completed a season-long loan move to newly-promoted
La Liga side
Real Valladolid. Hein and Valladolid started the season with a clean-sheet and a 1–0 victory over Espanyol.
Loan to Werder Bremen On 22 August 2025, Hein completed a season-long loan move to
Bundesliga side
Werder Bremen. On 26 September, Hein made his Bundesliga debut for Werder Bremen in a 4–0 away defeat to
Bayern Munich. Despite conceding four goals, he was praised for his performance, with Bremen’s managing director
Clemens Fritz describing it as “top-notch” and the club captain
Marco Friedl naming him their best player on the day. ==International career==