Julia Roberts' dog once was lost, but now is found and back in her yard ... thanks to her neighbor and a mobile app that connects everyone in her hood.

Someone in Julia's Malibu community found her chocolate lab, Major, earlier this month at Little Dume Beach near Point Dume in Malibu ... and then posted the news using Nextdoor -- an online social network specifically for neighbors.

The post read "Found Chocolate Lab," which was listed in the Lost & Found section of Nextdoor's Point Dume page. Sure enough ... Julia's husband, Daniel Moder, responded and got the contact info of the person who found Major. 

A neighbor tells us the pup was safely returned. Happy ending for everyone. Woof!