The occasion: Ben's 44th birthday.
According to Us Weekly, the family visited Montana's Yellowstone Club on Aug. 15 to celebrate the actor's birthday. The remoteness of the location helped keep news of their family gathering secret.
"There was a cake and they did the usual stuff: hiking and playing outside," a source told the mag. "They wanted to be together as a family."
The Montana resort, the source added, is one of Ben and Jen's "favorite places" -- they spent last Christmas there and they also vacationed there with friends Tom Brady and Gisele Bündchen in February.
Although the estranged couple is often seen together with their three kids, Violet, 10, Seraphina, 7, and Samuel, 4, the mag said they went their own ways as the night waned down.
"They didn't sleep in the same room, and they are not getting back together," according to the source, who added that they "got along great" during the birthday retreat.
The fact that Ben and Jen were together for his birthday is nothing new. Last summer, just two months after the duo announced plans to divorce, the family all together for the "Gone Girl" star's birthday at the Wizarding World of Harry Potter theme park in Orlando's Universal Studios.
Since then, the family have also been spotted in London, where he is currently shooting "Justice League," as well as Paris and Venice. They've also lauded each other for their careers and for their parenting in multiple interviews.
"I am a giant fan of Jennifer," Ben told CBS in March. "She's just a fabulous person. She's just a wonderful person. She's a great mother. She's a real talent."
Considering all the time the family spends together and the glowing praises, there have been rumors of a reconciliation, despite friends of the couple saying otherwise.
"This situation is working for them," a source said.
In that same CBS interview, the actor said, "Life doesn't always turn out exactly the way you want. We're doing our very best and we're putting our kids first. That's how we're focusing on our day-to-day lives, and we don't know what the future's going to hold.
"Each step that we take is one where we prioritize our children, and everything else comes second. She's just so good at doing that and sets such a good example. And that's a lead that I follow."