"Guesssss what?????? #Hedidgood," Spears, 21, posted Saturday evening, accompanied by a photo of the couple.
Spears and Watson, 30, have been dating on and off for three years, People magazine reported.
She had previously been engaged to Casey Aldridge, the father of her 4-year-old daughter Maddie, but that was called off in 2009.
Spears, the younger sister of pop star Britney Spears, and her daughter split their time between Nashville and Kentwood, La., Watson told the New York Post in January.
"Maddie goes to school in the Kentwood area, so Jamie Lynn tries to be in Louisiana most of the time," he said.