Chew, a Chicago-area native, was added to Illinois coach John Groce's staff in April. Chew previously had been at Missouri for one season before taking the Illini position.
Chew will replace Aki Collins as an assistant on Marquette coach Buzz Williams' staff.
"I really appreciate the opportunity coach Groce gave me," Chew said by phone. "This wasn't my plan. I didn't seek this out. I think coach Groce is going to do great things at Illinois. This is just a situation I can't pass up for me and my family. It was a family decision.
"There were some factors involved that were important to me as well as my wife. As coaches, we often make decisions. We always tell our wives to make sacrifices. This is an opportunity for both of us. She really felt great about the decision."
Chew understood his decision didn't come at a perfect time for the Illini as they try to rebuild the program.
"This is an emotional decision," Chew said. "I feel bad because it is terrible timing. I can't change that. Obviously, I do believe in coach Groce's vision."
Groce will now have a position to fill on his staff. Dustin Ford and Jamall Walker, who were assistants to Groce at Ohio, were named to Illinois' staff prior to Chew's hiring.