Sky Brown poses with her skateboard at a park in Kijo town, Miyazaki prefecture.
The pint-sized YouTube smash, born to a Japanese mother and a British father, told AFP she had taken the decision because Britain offered a more relaxed approach, given her tender years.
"I chose to compete for Britain because they told me 'Have fun, there's no pressure!' -- and I feel like that's the best way for me to skate my best," said Brown, the world's youngest professional skateboarder.
"Coaching is good and I want to work hard and try my best, but I mostly skateboard for fun -- it's my happy place."
Japanese skate officials struggled to match Britain's technical know-how and structure, leaving Brown to make a difficult choice -- and one which could potentially cost the Olympic host nation a medal.
"It would be really cool to qualify," said Brown, who is also a professional surfer and has just become Nike's youngest athlete.
"It's really cool that the Olympics will be in Japan because it's my home, it's where I was born.
"It will be super-special."
Brown will hope to become Britain's youngest-ever summer Olympian at the 2020 Tokyo Games at just 12.
Videos of Brown, who began skating before she could walk, have gone viral with over 50 million views, and her Instagram account already has more than 300,000 followers.
The freckled schoolgirl with sun-bleached hair landed her first major sponsorship deal aged seven and has perfected tricks no other woman has pulled off.
Sky Brown shows her skateboarding skills
"I skate a little differently than all the other girls," blushed Brown, who won a World Cup event in Estonia last month. "I wanna do the tricks that boys do.
Despite her prodigious talent, the teeny boarder, who stands just 1.34 metres (4ft, 4ins), has faced resistance from some event organisers reluctant to allow her to compete and risk embarrassing established skaters.
When she was eight, Brown became the youngest skater to compete at the Vans US Open, placing higher than women old enough to be her mother.
But Brown has set her sights on Tokyo 2020 when the hipster sport makes its Olympic debut, along with surfing, karate and sports climbing.
"I also want to show girls that it doesn't matter how old you are, you can do anything!
The qualifiers for the sport at the Olympics have not yet been set, but skateboarding's governing body has imposed no Olympic age limit.
"If I could win gold that would be pretty cool," said the multi-talented Brown, who last year won the junior competition on American hit show "Dancing with the Stars" and has just made her catwalk debut in Paris.
"But it doesn't really matter -- I'll just be glad to go to the Olympics and I'll feel good because I'll be the youngest there," she added.
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