"Very few people know and she isn't wearing her ring publicly yet," a friend of the couple told the celebrity magazine. "Looks like a summer wedding!"
Panettiere, 23, and Klitschko, 37, broke up in May 2011 after dating for two years. They reconciled in January.
UsMagazine.com didn't say exactly when the couple became engaged.
Notable deaths of 2014 [PHOTOS]