Steffen played his last game for the Crew on Saturday, as he will represent the U.S. national team at this summer's Gold Cup.
It is expected that he will then head to Germany to spend the 2019-20 season at Fortuna, who have sent representatives to Washington D.C to finalise a deal while the USMNT prepare to face Jamaica in a friendly in the capital.
Steffen began his career in the States but moved to Freiburg in 2014 and made his senior debut with Freiburg's reserves, before moving back to Columbus in 2016.
He was named MLS goalkeeper of the year in 2018 and was snapped up by City soon afterwards, although it was always planned that he would go out on loan.
“Columbus has been an amazing time and a crucial part of my career,” the keeper told mlssoccer.com last week. “It’s really the start of my career, and it’s been great. I’ll definitely make time to reflect when I’m settled over in Europe.”
Ahead of his final outing with the Crew, a 2-2 draw with New York City, Steffen paid tribute to the club's fans.
“They saved the Crew, they supported us through thick and thin, and they’ve been there every step of the way,” he added. “Without them, we wouldn’t be where we are today. They’ve given me so much confidence and motivation and passion to strive for greatness. I’ll never ever forget my time here or the people here.”
City will go into next season with Ederson as the club's No.1 goalkeeper, with 20-year-old Aro Muric battling with Chile veteran Claudio Bravo for a place on the bench.
Bravo missed the vast majority of City's domestic treble-winning campaign with an Achilles injury and has one year left on his contract, but is keen to stay at the Etihad Stadium as his family are settled in the area and he is unlikely to have his current wages matched elsewhere.