The National Football League, signaling a major shift in strategy, will stream live broadcasts of Sunday night football games beginning in September, making the contests widely available on the Internet for the first time.

For much of its history, the NFL has kept a tight grip on the rights to its games and the use of its images. But with more consumers, particularly younger viewers, using computers for entertainment, the NFL wants to steer the nation’s most popular television sport into the digital age.

“We are taking a big leap here,” said Steve Bornstein, chief executive of the NFL Network. “We are looking at this as a learning opportunity to see what applications work online. We are trying to be innovative and creative to make the viewing experience better for our fans.”

He said the league is interested in learning what kind of elements score best with viewers. All programmers, from the major Hollywood studios to mom-and-pop producers of two-minute “webisodes,” are grappling with how to make money online.

NBC, which broadcasts “Sunday Night Football,” will make its television feed — including Al Michaels’ play-by-play and John Madden’s commentary — available on Web sites run by both the network and league. The Web sites will offer blogs, statistics and other interactive elements along with the broadcast of the game.

NBC will sell advertising for the venture and share the revenue with the NFL.

NBC declined to comment. The NFL would not discuss financial terms of the deal. Bornstein called the venture a one-year “experiment.”

The online streams will begin with the Sept. 4 season opening match between the Washington Redskins and the New York Giants (which falls on a Thursday) and regular season Sunday night games. Playoff games and the Super Bowl will not be offered online, nor will the regional games televised by Fox Broadcasting and CBS.

Until now, the NFL has proceeded cautiously into the digital world, in part, to avoid antagonizing the networks that collectively pay the league $3.7 billion a year in fees for the exclusive rights to carry their games.

“The NFL’s most important constituency has been the television networks, but the world is moving online,” said Bobby Tulsiani, an analyst at the market research firm Jupiter Research. “They haven’t wanted people to watch games online because that could mess up their television deals. … This is going to get interesting as they move forward.”

In a recent report, Tulsiani found that nearly 20 percent of online users who considered themselves “serious sports fans” were young men, typically between the ages of 25 to 34. That’s a group that many marketers pay premiums to reach.

“With young men leading the pack in video activity, and with sports content translating well into the short-clip “snacking“ environment of web video, sport sites have a perfect opportunity to emerge as leaders in online video,” Tulsiani wrote.

Online streams of Sunday night games could ruffle the feathers of the peacock network’s traditional partners, the television station groups that carry NBC programming.

NBC executives recently told their affiliate station partners that they would be asked to help the network pay for its programming when their affiliation agreements come up for renewal.

NBC spends $600 million a year for the rights to broadcast Sunday night games, which doesn’t make a profit for the network, according to people familiar with the situation. Nonetheless, it represents the network’s biggest expenditure for a single program.

Television station owners have worried that they might be losing their place in the industry food chain as the Internet becomes a potent distribution outlet. In recent years, TV networks have tried to see whether offering popular shows online would cut into their audience and dampen ratings.

The industry will be watching to see whether online streams of NFL games cannibalize the enormous audiences that faithfully tune in to watch the games on TV. However, the NFL expects that many viewers will prefer to watch games on high-definition television sets rather than on computer screens.

“Many of our fans watch games on television, and they are on the computer at the same time,” Bornstein said. “We think the online streams will be additive and complementary.”

The NFL is not the first sports league to try the Internet. Major League Baseball, for example, has wired ball parks with digital cameras and offers a subscription service for fans who want to watch the games of out-of-town teams online.

Last season, DirecTV’s subscribers who paid extra to receive a premium package of Sunday football games also had access to online streams of those games.

“It was really for the convenience factor, when people were watching at the office or were over at their mother-in-law’s,” said Eric Shanks, DirecTV’s executive vice president for entertainment. “People think it’s cool. It has been a great add-on, but we don’t think that we’ve acquired new subscribers because of it.”