"I've committed to play football at Iowa State University! Truly thankful for the opportunity to play college football! Go Cyclones!!!!," McMillen said.
The 6-foot-2 McMillen is in Ames for Iowa State's first Junior Day.
McMillen's been an ISU lean since he picked up an offer in early February but was going to delay a decision until his his father, Bob, head coach of the Arena League's Los Angeles Kiss, could accompany the family on a visit to Ames later in the spring.
"No, I couldn't [wait]," McMillen told AllCyclones.com.
McMillen's tweet came shortly before Iowa State's basketball game against Oklahoma State. All J-Day visitors are at Hilton Coliseum.
Illinois was showing McMillen a lot of interest recently and Central Michigan had already offered.
AllCyclones.com and Scout.com will have more later on McMillen.