Spokesman Shin Inouye told the Chicago Sun-Times the president contended it was wrong to bar loving couples of whatever persuasion from tying the knot.
"Were the president still in the Illinois State Legislature, he would support this measure that would treat all Illinois couples equally," Inouye said.
The Legislature will take up the Religious Freedom and Marriage Fairness Act in Springfield as early as this week, the Sun-Times said. The Democratic sponsors of the measure hope to get the act to a vote during the January lame-duck session.
The newspaper said the bill's passage would likely require support from some Republicans even though Democrats control both houses in Springfield.
Obama rarely weighs in on Statehouse issues; however he was more willing to urge various states to open the door to same-sex marriage during his re-election campaign.