Janis Jonathan Blaswich was born on 2 May 1991 in Willich, Germany. He developed through Borussia Mönchengladbach's youth system but spent years in lower divisions before establishing himself, including loans at Dynamo Dresden and Hansa Rostock.
In 2018 he joined Dutch club Heracles Almelo, where over four seasons he made 107 senior appearances. In February 2022 RB Leipzig signed him as a backup, and his breakthrough came when he replaced the injured Péter Gulácsi during a Champions League game against Celtic on 5 October 2022. He seized the opportunity to become Leipzig's primary goalkeeper for the rest of that campaign and won the 2022-23 DFB-Pokal and the 2023 DFL-Supercup.
On 11 August 2025 Bayer Leverkusen signed him on a two-year deal for a reported €2 million plus €1 million in potential add-ons.
He earned a senior Germany call-up under Julian Nagelsmann in November 2023.
A 1.93 m goalkeeper with strong reflexes and confident long distribution, Blaswich is the kind of experienced senior No. 2 / No. 3 keeper that Bundesliga title contenders rely on for the long, demanding 50+ match seasons across multiple competitions.