The move was more or less expected for Steffen, who just finished helping the US men's national team to a runner-up finish in the Concacaf Gold Cup this past weekend, ever since The Athletic's Paul Tenorio first reported it last month.
In Dusseldorf, Steffen joins a club that enters its second season since its most recent promotion to the German top flight, after finishing in the middle of the table (10th place) over the 2018-2019 campaign.
Manchester City's agreement to sign Steffen from Columbus was announced last December, with the stipulation he would leave the club during the summer transfer window.
The 24-year-old leaves after 76 overall appearances for the Crew, including four clean sheets during 13 starts this season. He also preserved clean sheets in three of his five Gold Cup matches for the US — his first major tournament as the American starter — while allowing only two goals in total.