"Hillary is smart, she's determined, and most importantly, she is a champion for the American people," he said in a statement. "I'm proud to make my support for her official, and I'm even prouder to join more than two million Americans encouraging her to run."
Emanuel will host two fundraisers for Ready for Hillary in June, the group said.
The Chicago mayor becomes one of the most high-profile links between the White House and Clinton 2016 camp. He served as a top adviser to President Bill Clinton and Obama's first chief of staff.