It came in response to death threats made against the feminist reviewer Anita Sarkeesian after she released the latest in a series of videos on misogyny in gaming.
She said she contacted police and felt compelled to leave her home. She called the threats a "form of terrorism".
The letter called for gamers to report "hateful, harassing speech".
It read: "We believe that everyone, no matter what gender, sexual orientation, ethnicity, religion or disability has the right to play games, criticise games and make games without getting harassed or threatened. It is the diversity of our community that allows games to flourish."Public stand
The letter, which was written by games developer Andreas Zecher of independent games studio Spaces of Play, encouraged people who witnessed threats of violence on sites such as Twitch, Facebook and Twitter - as well as others - to report them.
"If you see hateful, harassing speech, take a public stand against it and make the gaming community a more enjoyable space to be in," it read.
It was signed by developers from major studios, such as Electronic Arts and Ubisoft, as well as smaller, independent ones.
On 25 August, Sarkeesian, who runs the Feminist Frequency website, released the latest episode in her series on the portrayal of women in video games.
In it, she addressed the tendency towards "largely insignificant non-playable female characters whose sexuality or victimhood is exploited as a way to infuse edgy, gritty or racy flavoring into game worlds".
Within two days, she said she had received "some very scary threats" against herself and her family on Twitter, adding that she had contacted the police.'Not giving up'
She also produced a screengrab that suggested the person who threatened her knew where she lived and said she was going to stay with a friend because she no longer felt safe in her own home.Continue reading the main story
? 55% of UK gamers are male
? 45% female
Source: Ipsos Mori
Later, Sarkeesian tweeted that she was safe. "I'm not giving up. But this harassment of women in tech must stop," she wrote.
And she suggested that the abuse was still going on in a tweet posted on Monday.
She has been subject to misogynist abuse before, most notably in 2012 when a first-person game was published online entitled: Beat Up Anita Sarkeesian.
Earlier this year, Sarkeesian won the Game Developers Choice ambassador award. The accolade is given to a person deemed to have "helped the game industry advance to a better place, either through facilitating a better game community from within, or by reaching outside the industry to be an advocate for video games and help further our art".