Jason Ritter's House Hit by Burglar While Fiancee was Home Alone

Jason Ritter's the latest celeb to get hit by a burglar, and this one was especially horrifying ... for his fiancee.

Law enforcement sources tell TMZ ... Jason's fiancee, Melanie Lynskey, was home alone last weekend in their Hollywood Hills crib when she heard some noise in the back of the house. We're told she thought it was Jason coming home, but then realized the back door window had been broken ... and it became very clear what had just happened -- a break-in.

Our sources say the burglar walked in through the door after breaking the window, but was apparently scared away after hearing Melanie, who then called 911.

Cops showed up, but the perp was gone by the time they arrived. LAPD's now investigating the attempted burglary. Jason -- son of the late John Ritter -- is just the latest celeb to be targeted by burglars. Rihanna's crib was hit a few days later in the same neighborhood.