Sources with direct knowledge tell TMZ ... the actor proposed to his girlfriend -- ABC7's Brianna Ruffalo -- on their recent trip to Italy, a pretty dang romantic place to bend the knee, we gotta say.

We're told the lovebirds traversed all over the country ... visiting Positano and Naples for a little summer R&R -- and apparently JL felt it was a perfect time to propose since Ruffalo's ancestors came from Italy.

Spoiler alert ... Ruffalo said yes and the happy couple got to tell their families the good news when they arrived in L.A. We're told the families are so excited for the couple.

BTW ... BR decided to play a cute little trick on her fans -- posting a video to social media from the back of a Vespa earlier this week -- and, if you pull out the magnifying glass, you can see her wearing the ring in the side mirror's reflection.

It's the culmination of a year and a half of dating that sources say kicked off when they met in a restaurant while Lucas filmed his new show "Palm Royale" costarring Kristen Wiig, Allison Janney, Carol Burnett and more bold-faced names.

This will be Josh's second marriage ... he was previously married to Jessica Ciencin Henriquez from 2012 to 2014.