"Guesssss what??????" the actress tweeted on Sunday, along with a photo of her posing with now-fiancé Jamie Wilson. "He did good."
The pair started dating after Jamie Lynn broke off her relationship with daughter Maddie's father, Casey Aldridge, in 2010.
And big sister Britney is thrilled, too.
"So excited and happy for my sister," she tweeted on Mar. 3. "Congrats @jamielynnspears - love you."
Jamie Lynn, 21, might be a little young to get married, but plenty of celebs married at a young age. Of course, those relationships had mixed results.
Actress Hilary Duff was only 22 when she got engaged to hockey player Mike Comrie in 2010. The pair later married and welcomed a child in 2012, Luca Cruz. They seem to be very happy in their relationship, though it seems like Duff never expected to get married so young.
"I think a reason that people in this business get married young is that they feel like everything comes to them sooner in life. It's normal to be like 'Okay, what's next?'" she told Cosmopolitan before getting engaged.
Miley Cyrus was only 19 when she got engaged to her "Last Song" co-star, Liam Hemsworth. The pair - dating for more than two years - isn't too concerned with the actual wedding part of the engagement, though.
"Life is too short not to be with the person you want to be with," she told Marie Claire in 2012. "I don't really care about the wedding or the piece of paper as much as I do the promise we've made to each other. And we want to have a long engagement."
Or, maybe they just need some time to grow up, first.
Avril Lavigne was both married and divorced by the time she turned 24. The "Boyfriend" singer married Sum 41 singer Deryck Whibley in 2006 at 19 and divorced in 2009.
"She married too young and she finally realized that," a source said at the time. "Avril realized she needs her own life and needs to explore things without him."
She went on to date reality star Brody Jenner before getting engaged to Nickelback lead singer Chad Kroeger in 2012.
Tell us: Do you think young Hollywood marriages ever work out?
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