Advertisers will finally be able to buy ads in YouTube clips of "The Tonight Show Starring Jimmy Fallon."

NBC Universal has struck a deal with YouTube that will add pre-roll video to clips of its content that appear on the site, the companies said on Wednesday. Until now, NBC clips have played on YouTube without any commercials, leaving marketers out and dollars on the table whenever one went viral.

NBC Universal will be responsible for selling the inventory. The companies declined to disclose how revenue will be split between them.

"We have been trying to figure something out for quite some time," said Linda Yaccarino, chairman-advertising sales and sponsorships, NBC Universal. "It's the next logical step and makes sense for YouTube to be our next distribution partner."

It's a meaningful development for NBC's late-night programming in particular. A clip of Mr. Fallon's "Lip Sync" battle with Emma Stone, for example, has been viewed more than 60 million times on YouTube, yet neither NBC or advertisers were able to capitalize on those views.

NBC hoped Mr. Fallon's presence on YouTube, where his page boasts over 8 million followers, would at least attract new audiences to "The Tonight Show" proper, but many of its YouTube viewers aren't necessarily big consumers of traditional TV.

Earlier in the year, NBC Universal CEO Steve Burke said that about 70% of the views of "The Tonight Show" occur online and "the majority of those views are un-monetized."

Ms. Yaccarino said YouTube viewing extends the reach for Mr. Fallon by about 20%.

This deal has been a long time in the making, as TV networks have struggled on reaching terms with YouTube in the past.

"This really demonstrates two partners over a period of time taking a bunch of whacks to hash out a deal," Ms. Yaccarino said.

CBS already has a deal in place with YouTube to make money from its clips through advertising, and ABC recently struck a deal with YouTube to do the same.