Marshall, 24, was out of contract and had been on a trial with Mainz of the German second division as negotiations continued.
"The base salary we offered was probably comparable to over there," Crew technical director Brian Bliss said. "I'm sure their bonuses were significantly higher, especially if the team is promoted, but he felt comfortable here with our offer and where he's at in this point of his career."
Crew president and general manager Mark McCullers said: "Chad was an anchor of our defense. I am a believer, although we scored a lot of goals this year, that defense wins championship and we were very good defensively in large part due to Chad.
"It's great we're going to keep our backline together. We're going to keep the spine of our team together with the exception of Evans. I feel awfully good about it in terms of keeping this team together and giving us the best chance to repeat."
McCullers added: "He's expressed a desire to go to Europe and at the right time and right situation he will go."