Though the MTV reality series ended abruptly in 2010, Port, 26, and Rosenman "kept in touch and have always had crushes on each other," a source explains. "Now they are full-blown together."
The two didn't begin dating until months after Port ended her longterm relationship with The Buried Life's Ben Nemtin, 29, in June 2011.
The source adds that Port and Rosenman -- who vacationed together in Miami on January 22 -- have become quite "serious."
Port, who will unveil her Fall 2012 Whitney Eve designs at New York Fashion Week Wednesday, was previously linked to Tamarama singer Jay Lyon.
- Whitney Port