According to Goal's Yves Galarcep City will pay $7.5 million (£6m) for his services, and the transfer fee could rise to $10 million, making him the most expensive MLS goalkeeper to date.
The 23-year-old is unlikely to secure a work permit for the United Kingdom and will almost certainly be sent out on loan. Options include Girona in Spain and Nice in France, where former New York City FC manager Patrick Viera is now in charge.
Steffen enjoyed a breakout campaign in 2018 and won the MLS Goalkeeper of the Year award while also putting himself in pole position for the starting spot with the national team. He faces strong competition from Club Brugge's Ethan Horvath, who himself is in the middle of a breakout campaign, and the move to Europe should help Steffen's case.
MLS' official YouTube channel had a debate whether he's the future for the Stars and Stripes:
U.S. Soccer also ran a feature about the youngster:
City have made several investments in American talents the last few years, signing the likes of Jack Harrison and Erik Palmer-Brown.
They also signed Irish star goalkeeping prospect Gavin Bazunu in October, according to the official website of his current club, Shamrock Rovers.
The Sky Blues' current starter in goal is Ederson, who at the age of 25 is just two years older than Steffen. The American faces serious competition to make his mark at the Etihad Stadium, but the lofty transfer fee indicates City believe in his potential.
He'll likely have the time to hone his skills on loan. He previously played for Freiburg but returned to the United States in 2016.