"Things have gotten more serious over the last month," a source tells PEOPLE. "They're perfect for each other."
The American Idol host and the former Dancing With the Stars pro have tried to keep things quiet, but In recent weeks, they've been spotted getting cozy all over Los Angeles.
On June 13, the couple walked on the beach in Malibu with their arms around each other. Three days later, they dined at intimate new L.A. hotspot Red O restaurant, where a diner says Seacrest, 35, "was very attentive." And Sunday, Hough, 21, accompanied the American Idol host on another outing in Malibu, where the two shopped at Alice + Olivia.
Hough, who split from country star Chuck Wicks last year, was also linked with comedian Dane Cook in March.
Reporting by JENNIFER GARCIA