"Guesssss what??????" Spears, 21, tweeted Saturday night, posting an Instagram photo of herself and Watson, 30, in a smiling embrace as she held up her left hand bearing an engagement ring.
She quickly posted a second image with a close-up of the large diamond ring and its jewel-encrusted band, tweeting, with a smiley-face emoticon, "He did good."
Britney Spears, 31, had not tweeted in response as of late Sunday afternoon; her most recent tweet was posted Feb. 28.
This would be the first marriage for the younger Spears sibling, best known for her starring role as Zoey Brooks in the 2005-08 Nickelodeon boarding-school comedy "Zoey 101."
Jamie Lynn Spears and Watson have been a couple on and off for three years, according to reports.
Spears has a 4-year-old daughter, Maddie Briann, with whom she lives in Kentwood, La., where her parents reside. Maddie's father, Casey Aldridge, then 18, had proposed to Spears in March 2008, shortly before Spears' 17th birthday. Spears and Aldridge broke up in 2010 without having gotten married.
Older sister Britney has been married twice, first to childhood friend Jason Alexander (not the "Seinfeld" actor), annulling it after 55 hours, and then to dancer Kevin Federline, with whom she has two young sons.
The siblings also have an older brother, Bryan.