The two reportedly became friends while appearing together in the off-Broadway production of The Way We Get By in 2015, and their relationship apparently turned romantic while shooting their upcoming film The Last Word.
The co-stars walked the red carpet together at the Tony Awards in June, where they appeared cozy and friendly.
Currently, Sadoski is in the middle of a divorce from his ex, casting director Kimberly Hope. Sadoski and Hope tied the knot in 2007 after dating for over eight years. Divorce proceedings began in October.