Both of their camps might stay mum on the subject, but that's when family members come in handy.
Of course, we're hearing this after Timberlake's obviously elated granny, Sadie Bomar, spilled the news to Gossip Cop...
"He called me on Christmas Day and told me he's engaged," Bomar told the website, explaining that J.T. asked her not to discuss the news with anyone "until after the holidays," and also shared that, "the family loves Jessie. She's a very sweet girl."
The joyous moment (which reportedly included a "custom-designed" ring) was certainly a big deal for everyone gathered in Big Sky Country to celebrate.
Timberlake "flew people out there to celebrate," says our source. "The engagement part was pretty emotional. And he got a little teary when making a toast."
No wonder Grandma couldn't wait to spill the beans!
This isn't the first time Granny Sadie served as a reliable mouthpiece, either. Back in October 2009, when breakup rumors were all over the Internet, she explained what was really going on.
"Jessica was keen to marry, but Justin isn't ready," she told The Daily Mail, "He's busy writing new material and he works hard to be successful, he's really focused on that. Justin does his own thing and he isn't ready to marry just yet."
We guess timing really is everything.