Harry Potter author J.K. Rowling has marked the 20th anniversary of the launch of her first boy wizard book with a heartfelt thank you note to fans.

The writer shot to fame after Harry Potter and the Philosopher's Stone was published on 26 June, 1997, and in the two decades since, the seven-book franchise has sold more than 450 million copies worldwide, spawned a huge blockbuster film series, and inspired its own amusement park, making the once-cash strapped single mom a multi-millionaire.

Fans flocked to social media to celebrate the anniversary on Monday (26Jun17), and Rowling herself issued a sweet but brief note to devotees on Twitter, expressing her gratitude to readers.

"20 years ago today a world that I had lived in alone was suddenly open to others," she posted. "It's been wonderful. Thank you."

Actress Bonnie Wright, who portrayed Ginny Weasley in the Harry Potter movies, also took a moment to pay tribute to Rowling for creating such a magical world for fans to enjoy.

Bonnie tweeted, "She had a vision. Congratulations @jk_rowling #HarryPotter20".

Meanwhile, co-star Evanna Lynch appeared on Britain's Lorraine talk show on Monday to discuss how Rowling's fantasy stories changed her life before she was even cast as Luna Lovegood in the film adaptations.

During the interview, the actress recalled how she became unlikely pen pals with Rowling as she battled an eating disorder in hospital.

"I was sick," she explained. "I had an eating disorder at the time and I wrote to her because I felt like Harry Potter was the only thing that took my mind off it.

"It was such a gift. And Luna especially inspired me because I think a lot of my problems were because I felt odd and felt weird and she (Rowling) made me see that that was OK and that was actually empowering me. And so I wrote to her and she wrote back and so we became friends, well pen friends, before I got the role."

Reflecting on the publishing anniversary, Lynch marvelled, "Twenty years... I was five! That's scary..."

"I was a big-time fan," she added of landing the part of Luna. "It didn't feel like a job, just like, 'Great, I get to play make believe.'"