A top prospect for the 2019 NHL Entry Draft, he was tabbed as "the best German defense prospect since Christian Ehrhoff" by The Hockey Writers.[2] On 21 June 2019, he was selected in the first round, sixth overall, by the Detroit Red Wings.[3] On 14 July, he signed a three-year, entry-level contract with the Red Wings.[4]
On 4 August 2020, Seider was loaned to his original club, Adler Mannheim of the Deutsche Eishockey Liga (DEL), to begin the 2020–21 season due to the delayed North American season as a consequence of the COVID-19 pandemic.[5] On 8 October, with no start date for the DEL in sight, the Red Wings terminated his loan with Mannheim.[6] It was announced on the same day that he would instead be loaned to Rögle BK of the Swedish Hockey League (SHL) for the remainder of the season,[7] joining former Adler Mannheim teammate Ben Smith.[8] After the 2020–21 season, Seider was named the SHL defenceman of the year.[9]
Seider made the Red Wings opening day roster out of training camp to start the 2021–22 NHL season.[10] Seider was named the NHL Rookie of the Month for October 2021 after recording eight points in nine games.[11] On 6 November 2021, Seider scored his first NHL goal, in overtime against Buffalo Sabres goaltender Dustin Tokarski in a 4–3 Red Wings win.[12] Seider scored seven goals and 43 assists during his first season with the Red Wings. He led first-year defencemen in assists, points, power-play points, game-winning goals and shots on goal, and ranked second in goals. On 21 June 2022, he was announced as the winner of the Calder Memorial Trophy, awarded to the player deemed the most proficient in his first year of competition in the NHL.[13]
In October 2021, Seider was named as one of three provisional players for Germany's Olympic roster for the 2022 Winter Olympics, alongside Leon Draisaitl and Philipp Grubauer.[17] However, the NHL ultimately decided against its players participating in the tournament.[18]
Seider initially declined to join the national team for the 2023 IIHF World Championship, but reconsidered in time to help lead them on an unexpected deep run through the knockout round, defeating the heavily-favoured United States senior team in the semifinal to reach the gold medal game.[19] Seider set up teammate JJ Peterka's opening goal of the game, but Germany was ultimately defeated by Canada senior team, winning the silver medal, the nation's first at the World Championships since 1953.[20] For the second time, Seider was named to the Media All-Star Team.[21][22]
