Santiago agreed to a minor-league contract with Chicago on Friday, Scott Merkin of MLB.com reports.
Santiago became a free agent Wednesday after being designated for assignment by the Mets, as his time on the open market didn't last long. He struggled to a 6.75 ERA with six strikeouts over eight innings this season with New York, and he'll report to Triple-A Charlotte after linking back up with Chicago -- the team he made his big-league debut with in 2011.