On Monday, Raiders defensive lineman Carl Nassib became the first active N.F.L. player to publicly declare that he is gay.

“I just want to take a quick moment to say that I’m gay,” Nassib said in a video posted to his Instagram account. “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, but until then I’m going to do my best and my part to cultivate a culture that’s accepting, that’s compassionate,” before adding that he would donate $100,000 to The Trevor Project, a nonprofit group that focuses on suicide prevention efforts among lesbian, gay, bisexual, transgender, queer and questioning youth.

“Sadly, I have agonized over this moment for the last 15 years,” he wrote in the same post.

Nassib, a five-year N.F.L. veteran who previously played with the Cleveland Browns and the Tampa Bay Buccaneers, said he was finally “comfortable getting it off my chest.”

Nassib, 28, thanked his coaches, teammates and the N.F.L. for their support.

“I would not be able to do this without them,” he wrote in his Instagram post.

In a statement Monday, Commissioner Roger Goodell said he was “proud of Carl for courageously sharing his truth today. Representation matters.”

The Raiders quickly showed their support for Nassib’s announcement, writing “proud of you, Carl” in a post to the team’s Twitter account that also included his original statement. Two of his teammates, defensive lineman Darius Stills and edge rusher Maxx Crosby, voiced their support by commenting under Nassib’s post that they were proud of him. DeMaurice Smith, executive director of the N.F.L. Players Association also said in a Twitter post that he and the union supported Nassib.

Nassib’s announcement, made during Pride Month, is a significant turning point for the N.F.L., and makes him the first openly gay active player in the league’s 101-year history.

“Sports are, in many ways, one of the last bastions of a place where homophobia can thrive,” said Cathy Renna, a spokeswoman for the National L.G.B.T.Q. Task Force. “So to have a professional athlete of that caliber, particularly in one of the major sports leagues like the N.F.L., it really is historic.”

A bevy of current and former athletes from around sports reacted positively to Nassib's announcement, including the retired tennis star Billie Jean King, who wrote, “the ability to live an authentic life is so important,” in a social media post Monday.

Sarah Kate Ellis, chief executive of the L.G.B.T.Q. advocacy organization Glaad, called the announcement “a historic reflection of the growing state of L.G.B.T.Q. visibility and inclusion in the world of professional sports, which has been driven by a long list of brave L.G.B.T.Q. athletes who came before him.”

Michael Sam, an all-American defensive lineman at Missouri, had been viewed as the most likely to acquire that distinction when he announced he is gay before he being chosen by the Rams in the seventh round of the 2014 N.F.L. draft, but he was cut at the end of that year’s training camp. The Dallas Cowboys signed Sam to their practice squad, but he never played in a regular season game.

Sam’s draft status was seen as a barometer of whether the climate of men’s pro sports was becoming more accepting of gay athletes, particularly because in February 2014 the N.B.A. had just become the first of the four traditional major American men’s sports leagues to have an openly gay active player when Jason Collins joined the Nets.

But Sam left the N.F.L. without making an impact on the field.

Nassib, by contrast, has already played with three teams over five seasons and is under contract through 2022. After a collegiate career at Penn State, he was chosen by the Browns in the third round of the 2016 draft. He played two seasons in Cleveland before playing two more seasons in Tampa. The Raiders signed him to a three-year, $25 million contract in March 2020. He has tallied 20½ sacks during his career.

A handful of N.F.L. players had previously announced publicly that they were gay, but all after their playing careers were over. David Kopay became the first pro football player to publicly come out as gay in 1975, three years after he retired. He played for nine seasons with the San Francisco 49ers and four other teams in the 1960s and 1970s, and has since become an activist and an ambassador for the Gay Games, a quadrennial sporting event.

Roy Simmons was the second former player to announce that he was gay, doing so in 1992 after his career with the Giants and Washington Football Team had ended. He later disclosed he was H.I.V. positive and died from pneumonia-related complications in 2014 at age 57.

Some players like Simmons said they felt they had no choice but to hide their sexual identity while they were in the league. Simmons said he cultivated a reputation for being the life of the party, and had to compartmentalize his football life and his personal life.

Simmons also said he never would have declared himself gay during the four seasons he played for the N.F.L. for fear of destroying his career.

‘’The N.F.L. has a reputation,” he said in 2003, “and it’s not even a verbal thing — it’s just known. You are gladiators; you are male; you kick butt.”

In recent years, the league has publicly supported Pride Month through promotional efforts like changing official social media avatars to include rainbows and supporting the You Can Play Project, which provides resources to encourage inclusivity in youth sports, even as some players have made derogatory statements about gay people with little penalty or supported groups that oppose gay rights.

In 2013, Chris Culliver of the San Francisco 49ers and Chris Clemons of the Seattle Seahawks made offensive comments when asked about the prospect of having a gay teammate.

“Got no gay people on the team,” Culliver said. “They gotta get up outta here if they do.” Culliver later apologized, saying: “I’m sorry if I offended anyone. They were very ugly comments.”

San Francisco running back Garrison Hearst apologized in 2002 for using a slur and saying he wouldn’t want a gay player as a teammate. His comment came after the former Minnesota Vikings player Esera Tuaolo publicly came out as gay that year after he had retired. Hearst’s comment elicited public apologies from the 49ers’ team owners and then-head coach Steve Mariucci, but no penalty from the league.

“Being an African American, I know that discrimination is wrong,” Hearst later said. “I was wrong for saying what I said about anybody, any race or any religion.”

The league had little to do with Sam’s announcement because it came before he was drafted. Former N.F.L. players like Brendon Ayanbadejo, who played with the Baltimore Ravens, defended same-sex marriage and gay rights and supported Sam at the time. But few active players publicly echoed his support.

Seven years after Sam’s announcement, Nassib’s announcement has been met with ready public support both from the league itself and the Raiders, a team that had previously made notable football milestones with its hires. Tom Flores, who is Mexican-American, was the first Latino coach in the N.F.L. to win a Super Bowl. He won two with the Raiders, in the 1980 and 1983 seasons.

Amy Trask in 1997 became the Raiders’ chief executive and the first woman of that rank in the N.F.L. The team drafted Eldridge Dickey, the first Black quarterback taken in the first round, in 1968, when the Raiders played in the A.F.L.

“We hope that Carl’s historic representation in the N.F.L. will inspire young L.G.B.T.Q. athletes across the country to live their truth and pursue their dreams,” Amit Paley, the executive director and chief executive of the Trevor Project, said in a statement Monday.