Dillon and Ronson - twin sister of DJ Samantha - were snapped enjoying dinner on 2 May (10) at Los Angeles restaurant Madeo.
The pair was first spotted together in February (10) at the designer's New York City fashion show.
And according to U.S. magazine Star, the duo have been dating quietly for months.
A source says, "He pursued her, and it really paid off."
Dillon has previously dated actresses Cameron Diaz and Eliza Dushku.