In a Twitter post, Sonny Bill Williams also criticised others for choosing "economic benefits over humanity".
Arsenal footballer Mesut Ozil recently spoke out on the issue.
China has been accused of a "mass internment" of one million Muslims in the Xinjiang region.
Williams, a Muslim convert from New Zealand, posted an image showing a hand painted with the Chinese flag twisting an arm painted with the flag of East Turkestan - the term used by separatists to refer to Xinjiang.
The East Turkestan arm is dripping with blood.
Williams, who recently signed for rugby league side Toronto Wolfpack, has achieved huge success in both codes of the game - league and union.
He also had a short but successful career in professional boxing.
His statement echoes comments made by Arsenal footballer Ozil who criticised both China - and people who stayed silent.
"Korans are being burnt... Mosques are being shut down... Muslims schools are being banned... Brothers are forcefully being sent to camps," the Muslim player tweeted in Turkish, to a background of the East Turkestan flag.
"The Muslims are silent. Their voice is not heard."
China responded furiously, with state television pulling an Arsenal match from TV schedules.
Ozil was also removed from a Chinese version of the Pro Evolution Soccer video game.
A foreign ministry spokesman said he was "deceived by fake news", and invited him to visit the region to "have a look" for himself.
After Ozil's comments, UFC fighter Khabib Nurmagomedov's Instagram account included a post about the history of Uighurs - but the post seems to have been taken down.
More than 20 countries, including Britain, signed a letter earlier in July condemning China's repression of Uighur Muslims.
The US House of Representatives has also passed a bill to counter what it calls the "arbitrary detention, torture, and harassment" of Uighurs.
The bill still needs approval from the Senate and US President Donald Trump.