"It is a special feeling to aim for the Olympics as a representative of Japan", she told NHK.
Japanese law states those with dual citizenship must select one before turning 22.
Two-time grand slam champion Osaka, who was born in Japan to a Japanese mother and Haitian father but was brought up in the USA, told NHK that it gives her "a special feeling to try to go to the Olympics to represent Japan". Osaka turns 22 on Wednesday.
Tennis star Naomi Osaka has decided to choose Japanese over American nationality with an eye on the Tokyo 2020 Olympics, public broadcaster NHK reported on Thursday.
Japan, which has long prided itself on being homogeneous, is becoming more ethnically diverse. Still, it will be hard for Osaka to totally be accepted, Reuters reported.
Mixed-race children can often face prejudice in Japan, although attitudes among younger generations are changing.
Two weeks ago, a Japanese comedy duo teased Osaka, saying she was "too sunburned" and "needed some bleach".
Osaka won the China Open on Sunday and is now the No. 3-ranked women's singles player in the WTA.
She also became the first Asian player to reach No. 1 in women's singles competition after the Australian Open victory. She is now ranked No. 3 in the world.