A source close to the couple told People.com: "Things have gotten more serious over the last month. They're perfect for each other."
In recent weeks the couple have been spotted getting cozy all over Los Angeles.
They were photographed walking arm-in-arm on a Malibu beach last week (13.06.10), and spotted again three days later (16.06.10) at a trendy restaurant.
Onlookers revealed that Ryan, 35, was "very attentive" of the 21-year-old beauty.
On Sunday (20.06.10), Julianne, accompanied Ryan on another outing in Malibu, where the two shopped at trendy boutique Alice and Olivia.
Julianne, who split from country star Chuck Wicks last year, was also rumoured to be dating comedian Dane Cook in March.