Republican Rep. Mark Kirk, a Navy reservist, had said he was given the U.S. Navy's Intelligence Officer of the Year award for his service during NATO's conflict with Serbia in the late 1990s, The Washington Post reports.
A complaint from a representative of Kirk's Democratic rival in the Senate race sparked an inquiry, the Post said.
On Friday, Kirk changed his biography on his Web site with a different account of the award.
"Upon a recent review of my records," Kirk wrote, "I found that an award listed in my official biography was misidentified."
The award he had intended to list, he wrote, was given to his unit, not to him individually.
A spokesman for Kirk's campaign had no comment except to say, "We found the award was misidentified and corrected the name."