The former Friends star was doing a test photo shoot in Chris McMillan's Beverly Hills salon when the "Leftovers" actor peeked in and said "hi."
"She clearly wasn't expecting to see Justin because she squealed when he showed up," an insider told Us Weekly. "They hugged and he gave her a kiss and then he watched the end of the job."
Sources told E! News that Theroux even jumped in to pose for a few photos with Aniston for the salon-themed shoot after the surprise. The pair later spent some time with the actress' trusty hairstylist.
Theroux, 42, recently opened up about his "kinda perfect" bi-coastal lifestyle, telling Interview magazine that while he loves his life in New York City, "I love someone who lives in Los Angeles -- so that's a big draw."
The Wonderlust co-stars have been engaged since August 2012. Before recently admitting that Aniston changed his life, Theroux told reporters in June that both he and his fiancee had "hot feet" about their upcoming nuptials.