The 29-year-old actress shared a photo Wednesday with former co-stars Tom Felton, Evanna Lynch, Bonnie Wright and Matthew Lewis.
"Merry Christmas from us," she captioned the post.
Felton posted a pair of similar photos on his own account.
"Seasons greetings from my school mates. @mattdavelewis & I mid debate x #gryfindorVSslytherin," he wrote.
Watson played Hermione Granger in the Harry Potter movies. Felton co-starred as Draco Malfoy, Lynch as Luna Lovegood, Wright as Ginny Weasley and Lewis as Neville Longbottom.
"I never believed the whole 'I'm happy single' spiel," she said. "I was like, 'This is totally spiel.' It took me a long time, but I'm very happy [being single]. I call it being self-partnered."
Rupert Grint, who played Ron Weasley, discussed Watson and Felton with Entertainment Tonight the same month.
"There was always something," Grint said of the pair. "There was a little bit of a spark."
"But we were kids. It was like any kind of playground romance," he added.