The True Blood star became engaged to the model in October 2010 after a year and a half of dating. However, the couple have ended their engagement, according to Us Weekly.
"It's totally over," a friend of the pair told the website.
Another source confirmed reports of the split, saying: "It was a long time coming. They'll both be better off for it."
In January, Manganiello shared his wedding plans, saying that the cast of True Blood would be invited to his nuptials.
The actor recently insisted that he is comfortable with being naked, claiming that he considers it normal to be in the nude on set.
True Blood concluded its fourth season on Sunday.