While cross-code player Gareth Thomas also came out back in 2009, Stanley is the first exclusively union player to do the same.
The 23-year-old, who also plays for the Ealing Trailfinders, is the nephew of New Zealand’s 1987 World Cup-winning centre Joe Stanley.
His brother Mike, meanwhile, will play for Samoa at the upcoming World Cup.
“I was 10 or 11 when I realised I was different to my friends,” Stanley told the Sunday Times.
“I didn’t want to accept it – I felt being different wasn’t right. I had a girlfriend and I was thinking that, like some people say, maybe it is a phase.”
Stanley’s revelation comes just one month after Batley Bulldogs prop Keegan Hirst last month became the first rugby league player to openly come out as gay.
“Millions of people are in the situation, even people who are still married but are gay. They cannot accept it themselves,” added Stanley.
“It is going to be an issue until more people and athletes come out, until it is not an issue at all. It might take years but hopefully lots of people will find the courage.”
Stanley’s brother Mike took to Twitter to congratulate him.
“So proud of my little brother,” he tweeted. “Not easy in this day’s society! Love you uce [Samoan slang for ‘brother’], be proud of who you are.”