The British author turned 54 on Wednesday (July 31), but celebrated a day early with some of the stars of the films based on her beloved wizarding books.
Watson revealed how much she had enjoyed Rowling's party in a post on Instagram, which showed her with the birthday girl and Lynch, all dressed in costume.
"Sexy and scary! (ghost emoji) You smashed this," she captioned the image. "All the love to you Jo. Happy Birthday!!!! Extra points to @msevylynch for being the most perfect cat. (tiger emoji)."
The snap shows "The Circle" star dressed as Wonder Woman, while Rowling looked glamorous and ghoulish in a black halterneck dress and gory make-up over one half of her face, to give the effect that a demon was emerging through her skin. Evanna donned a green dress and sparkly cat make-up, and completed her feline look by styling her blonde locks into ears.
Watson became close friends with the author during her childhood, after she was cast as Hermione Granger in the 2001 film adaptation of Rowling's first Harry Potter novel, "Harry Potter and the Sorcerer's Stone", and went on to star in seven more films in the franchise.
The pair remained friends after the release of the final movie in 2011, with Emma interviewing the novelist for Wonderland magazine in 2016 and the pair frequently championing each other's charity work.