Newly single Mellencamp dating Meg Ryan

Heartland rocker John Mellencamp, a week after announcing his breakup with his wife, has begun dating actress Meg Ryan, according to witnesses who have seen them together in Manhattan and elsewhere.

Grammy-winner Mellencamp, 59, whose hits include "Jack & Diane" and "Hurts So Good," and Ryan, 49, star of "When Harry Met Sally . . . " and "Sleepless in Seattle," were first spotted together in October, a source told Us magazine, adding that this would have been "well after" Mellencamp and his wife of 18 years, former Victoria's Secret model Elaine Irwin Mellencamp, 41, decided to split up.

Ryan and Mellencamp have been seen together on Martha's Vineyard, according to People magazine, as well as grocery shopping at Gourmet Garage in New York's SoHo neighborhood.

"She usually comes in by herself or with her daughter," Daisy True, 6, one observer told the magazine. "I thought [Mellencamp] was her husband, because they seemed so comfortable and familiar with each other, just like an old married couple."

Ryan has a son, Jack Henry, 18, with ex-husband, actor Dennis Quaid. Mellencamp has been married three times and has five children, including two sons with Irwin Mellencamp - Hud, 11, and Speck, 10.