The "Beetlejuice Beetlejuice" star proposed to Nicole in Italy ... according to PEOPLE.

Justin and his castmates are promoting their new movie at the Venice Film Festival ... so it looks like he took advantage of the overseas work trip to find just the right moment to propose. Savvy move.

The couple sparked engagement rumors Wednesday at the 'Beetlejuice' world premiere ... where Nicole was photographed with a big diamond ring on that finger.

Nicole didn't try to hide the ring at all ... proudly displaying it in photos with her boo. As she should -- the diamond looks massive.

Justin and Nicole were first linked together in February 2023 at a Netflix event in New York City ... but it wasn't until December that the pair went Instagram official.

If Justin and Nicole make it down the aisle, this would be his second marriage. Justin was previously married to Jennifer Aniston from 2015 until their 2018 divorce.