"They are dating," a source told the magazine. "They've been spending a lot of time together and are having fun. He showers her with attention and is helping her move on from her breakup with Jamie [Kennedy]."
Hewitt and Kennedy broke up in March after dating for a year. Asher and McCarthy were married from 1999 to 2005, and have one son, Evan, now 8.
Hewitt and Asher may have met on the set of "The Ghost Whisperer" when he taped a guest appearance for an upcoming episode earlier this year.
People's source declined to confirm how the two met or when they began dating. A rep for Hewitt had no comment on the possible relationship.
McCarthy also recently went through a breakup - on April 6, she and actor Jim Carrey announced they had parted ways after a five-year relationship.