Japanese tennis player Naomi Osaka has added her voice to protests over the death of George Floyd, criticising people for tweeting more about lootings than his death.

Protests set off by the death of George Floyd, an unarmed 46-year-old who died in Minneapolis after being pinned beneath a white police officer's knee for nearly nine minutes erupted around the United States again on Monday, the latest of several days of violence.

Osaka, who was born in Japan to a Haitian father and Japanese mother, has previously passed her time in quarantine posting lighthearted selfies in stylish clothes or by the side of a pool.

But her social media feed changed following Floyd's death, as she posted news footage of his death and the statement "There comes a time when silence is betrayal" as well as tweeting "Just because it doesn't happen to you doesn't mean it isn't happening."

"When you tweet about the lootings before you tweet about the death of an unarmed black man," she posted on Monday.

On Facebook, she posted a photo of a wall painting of Floyd's face, as well as several others from a protest under the hashtag "justiceforgeorgefloyd".

Osaka joins other sports figures in speaking out against racism and police violence, including basketball great Michael Jordan and Formula One champion Lewis Hamilton.

Osaka has mostly sidestepped strong responses to controversy in the past, including last year when a Japanese comedy duo said she was "too sunburned" and "needed some bleach."

Instead, she laughed it off, turning it in to a plug for sponsor Shiseido.

"'Too sunburned' lol that's wild," she said on Twitter. "Little did they know, with Shiseido Anessa perfect UV sunscreen I never get sunburned."