Nassib, 28, made the announcement in a video on Instagram Monday, where he also said he is donating $100,000 to the Trevor Project, which is the nation's largest organization for suicide prevention for LGBTQ+ youth.
"I've been meaning to do this for a while now, but I finally feel comfortable enough to get it off my chest. I really have the best life I got the best family, friends and job a guy could ask for," he said in the video. "I'm a pretty private person, so I hope you guys know that I'm really not doing this for attention. I just think that representation and visibility are so important. I actually hope that like one day videos like this and the whole coming out process are just not necessary."
Nassib added in a written message that followed the video that he "agonized over this moment for the last 15 years" and only recently decided to go public with his sexuality after receiving the support of family and friends.
"I am also incredibly thankful for the NFL, my coaches, and fellow players for their support," Nassib wrote. "I would not have been able to do this without them. From the jump I was greeted with the utmost respect and acceptance."
Shortly after the announcement, the Raiders organization released a statement, which read in part: "Nassib's announcement will go down as a historical one in the NFL and the world of sports, along with being a step forward in a path toward equality."
Sarah Kate Ellis, president and CEO of GLAAD, a leading LGBTQ advocacy organization, called Nassib's "powerful coming out is a historic reflection of the growing state of LGBTQ visibility and inclusion in the world of professional sports, which has been driven by a long list of brave LGBTQ athletes who came before him."
Ellis said Nassib's story "will not only have a profound impact on the future of LGBTQ visibility and acceptance in sports, but sends a strong message to so many LGBTQ people, especially youth, that they too can one day grow up to be and succeed as a professional athlete like him."
More than a dozen NFL players have come out as gay after their careers were over.
Former University of Missouri defensive star Michael Sam was the first openly gay football player ever selected in the NFL draft, going in the seventh round to the then-St. Louis Rams in 2014. But he never made the final roster and retired in 2015 having never played in an NFL regular-season game.
Nassib is a sixth-year pro who was drafted by the Cleveland Browns in 2016 in the third round (65th overall) out of Penn State. He played two seasons for the Browns and two for Tampa Bay before joining the Raiders in 2020. He has 20 1/2 sacks in 73 career games.
The Associated Press contributed to this report.