The British author has founded Lumos, an organisation which aims to help kids living in institutions across the world.
After donating almost $37.5 million (£25 million) of her personal fortune to the cause, Rowling threw a huge gala event at the Warner Bros. Harry Potter Tour attraction in Hertfordshire, England on Saturday (09Nov13).
Watson, who played Hermione Granger in the films, joined Rowling at the bash, alongside fellow castmember Evanna Lynch and fashion designer Stella MCCartney.
After the event, the author took to Twitter.com to thank her generous guests for helping to raise $1.5 million (£1 million) for the charity, writing, "Fantastic that we raised over £1M at (the) Lumos fundraiser - going directly towards ending the institutionalisation of children."
All proceeds from her recent Harry Potter spin-off book, The Tales of Beedle the Bard, will also go to Lumos.