Spears, 21, announced the happy news on Twitter, with an Instagram picture of herself flashing the ring and hugging her fiance, 30. "Guesssss what??????" she captioned the sweet shot.
Minutes later, she posted another photo: a close-up of the huge, round-cut diamond. "He did good," she wrote, with a smiley-faced emoticon.
This will be the first marriage for the former Nickelodeon star. She was previously engaged to Casey Aldridge, the father of her 4-year-old daughter, Maddie. They split for good in 2010.
Spears' engagement comes just two months after news of her older sister's broken engagement. Britney, 31 -- married twice before, to pal Jason Alexander and ex Kevin Federline, with whom she has two kids -- was set to wed boyfriend Jason Trawick until the couple's January 2013 split.
This article originally appeared on Usmagazine.com: Jamie Lynn Spears Engaged to Jamie Watson: See Her Ring!