Sources tell Life & Style magazine the actor's new girlfriend is Sarah Austin, the 26-year-old star of new series Start-Ups: Silicon Valley.
Grenier, 36, reportedly met Austin several years ago, when she interviewed him at the premiere for his documentary Teenage Paparazzo in her native San Francisco, California.
The pair became friends and recently lovers, according to the publication.
One insider says, "Things have gotten pretty serious in the past month. Right now it's long distance because she lives in San Francisco and he lives in L.A., but they are making it work."
Austin's new series about budding entrepreneurs begins on 5 November (12).