Correction appended on June 7, 2013

Though it’s now easy to consume music, movies and television on the Internet, watching live sports online is still a fantasy for many fans. Even as other forms of entertainment have digitized rapidly in recent years, sports for the most part have remained carefully locked in the walled garden of cable subscriptions and expensive all-access digital passes. But a surprising new player in sports broadcasting may soon change that: YouTube.

The video-sharing website originally famous for funny home videos is quietly brokering deals with leagues as varied as the NBA and the Badminton World Federation. It’s acquired a mixture of highlight reels edited by the sports leagues themselves, archival footage of historic games and live-streaming rights to a variety of events. Such deals could eventually place YouTube in direct competition with ESPN, Fox and the other sports-broadcasting heavyweights. That would spell trouble not only for them but also for the entire model of cable television, which has become reliant on a still thriving sports market to make up for overall declining interest in traditional television.

For now, the major American sports are taking small, experimental steps on YouTube. Major League Baseball is live-streaming two games per day this season on the video site outside major markets like the U.S., where the league has television deals. This past season the NBA live-streamed 350 games of its minor-level D-League and regularly posts SportsCenter-like highlight reels of nationally broadcast NBA games. The London Olympics were broadcast in several Asian and African markets, and the International Olympic Committee’s YouTube channel now hosts 12,000 hours of archival Olympics footage from past years.

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Smaller sports are being more aggressive and establishing themselves on YouTube with programming similar to sport-specific cable channels, just delivered via the Internet. The Ultimate Fighting Championship was one of more than a dozen brands to launch a pay-subscription channel when YouTube introduced the concept last month. UFC also charges for live-streaming individual fights, similar to pay-per-view on television. Willow TV, a cable channel focused on cricket, has a live-streaming package hosted on YouTube that costs $14.99 per month.

Why are all these leagues lining up to put their content on YouTube, when they could limit availability to still lucrative broadcast markets and their official league-owned websites? Because YouTube’s user base, now more than 1 billion, is massive — and not all its users can be reached on television. According to Nielsen data from March, YouTube reached more Americans between the ages of 18 and 34 than any cable channel, including ESPN. “The goal is to use this incredibly powerful name and platform to reach 10, 20, maybe even 100 million fans around the world that we wouldn’t talk to normally,” says Bob Bowman, CEO of MLB Advanced Media. “I think of my two teenage children. Sadly, they are much more likely to watch a baseball highlight on YouTube than they are on MLB.com.”

The site has proved that its young users have an appetite for live content as well. When Felix Baumgartner broke the world skydiving record in a YouTube live stream last fall, more than 8 million people tuned in simultaneously. That’s not Super Bowl–level ratings, but it’s a larger viewership than some NBA playoff games are getting this season in the U.S. Individual sports channels have also performed well. The NBA’s YouTube channel has more than 1.4 billion views, putting it in the top 40 most viewed channels overall. “It’s been something that enhances and drives tune-in [on television] for us,” says Melissa Rosenthal Brenner, the NBA’s senior vice president of marketing.

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Successes like the Baumgartner edge-of-space jump could lay the groundwork for a splashier initiative with one of the major American sports. The company has been aggressively pursuing premium content for the past couple of years. It invested a reported $100 million in original content across a variety of genres early in 2012, including about 17 sports-themed channels. Another $200 million was put toward marketing YouTube’s most successful channels last fall. And just last month, YouTube rolled out a line of pay-subscription channels, creating a straightforward way for content producers to offer TV-quality programming on an à la carte basis. “Certainly, YouTube’s infrastructure has shown that they can handle a live event in terms of millions of people [tuning] in,” says Mike McGuire, a researcher at Gartner. “Because they’ve now set up and created these dedicated channels, that makes them particularly scary to a lot of the conventional programmers.”

If YouTube does want exclusive broadcast rights to a big-ticket sport, it will be an incredibly expensive endeavor. Television networks pay around $1.5 billion annually to broadcast MLB games, $930 million for NBA rights and a whopping $3 billion for NFL football. These sports leagues have become the most valuable television properties in existence, not only because they bring in huge ratings but also because they convince millions of fans each month that they can’t cancel their cable subscription if they want to watch the big game. “One of the main reasons you see a lot of [customers] continuing to pay for the subscription packages is because of sports, live sports in particular,” McGuire says. “Those are probably going to be the last bastions of these controlled systems.”

At this point, YouTube’s not committing to a big-ticket play. The deals they have with sports leagues right now mainly center around splitting ad revenue, like most content on the site. Claude Ruibal, YouTube’s global head of sports partnerships, says the company is a friend to broadcasters, not a foe. “We’re a technology platform. We’re not a licensee of content,” he says. “That’s not what we’re good at. We’re good at distributing it, helping create an aggregated global audience.”

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Sports executives and media analysts say it’s premature to think about big-name sports jumping fully online, not only because of licensing issues but because of user access. Before a mainstream sport resides exclusively on a platform like YouTube, watching live online events on television sets will have to be just as easy as flipping to ESPN for most TV viewers. Such technology does exist, and more viewers are adopting it each year. “Smart TVs,” which have wi-fi capabilities and come installed with apps for Web services like YouTube and Netflix, are only used in 5% of American households, according to Nielsen, but market-research firm IHS iSuppli predicts 141 million of them will be shipped globally by 2015. Online-streaming devices like Roku and Apple TV, which allow people to view online video content directly on their television sets, have seen rapid sales gains in the past year without extensive marketing. Google has been quiet about its own set-top box, Google TV, but the device is getting an operating-system upgrade later this year.

Whether YouTube will play a supporting role in the move to online TV or offer game-changing exclusive content itself mostly depends on how deeply Google is willing to dig into its coffers. Sports leagues, for their part, aren’t married to the idea of beaming their games to fans through a cable box. MLB’s Bowman says he could see much more of his league’s content appearing on YouTube in the next few years, and an NBA official pointed out that some of the league’s past technological experiments, like the shot clock, began in the D-League. These sports want to be where fans are and — critically — where they’re going to be. “The technology is an 800-lb. boulder coming down the hill,” Bowman says. “All of us who publish content — whether it’s sports, whether it’s food, whether it’s music — have to think about how we reach people based upon where their habits are.”

Correction

An earlier version of this article incorrectly stated that Google TV’s last operating-system upgrade was in 2011. It was in 2012.