YouTube’s mobile ad revenue triples as mobile views reach 40% in the US

Mobile is quickly becoming the new normal for YouTube, (s GOOG) and it’s starting to pay off: 40 percent of all of the service’s video views in the U.S. are now coming from mobile devices, a YouTube spokesperson told GigaOM on Wednesday. Worldwide, mobile is now responsible for 25 percent of all of YouTube’s video views.

This, and the roll-out of new mobile apps, has helped YouTube to triple its mobile ad revenue in the past six months, according to Bloomberg, which got YouTube VP of Sales Lucas Watson go on the record about the mobile growth.

Bloomberg is also guesstimating that “as much as $350 million in sales probably came from mobile video ads” in Google’s last quarter alone. However, that number isn’t coming from Google, and it’s based on a few assumptions from analysts, so I would take it with a grain of salt.

Still, even without a firm mobile revenue number, tripling mobile ad revenue in a six-month time frame is impressive, and it seems to be the direct result of YouTube launching its iPhone and iPad apps late last year, as well as rolling out a full-featured mobile web experience across all platforms. Previously, iOS users were accessing YouTube through an app built by Apple, (s AAPL) and YouTube wasn’t able to monetize any of those views.

For more on YouTube’s mobile strategy, check out this interview I did with Shiva Rajaraman, YouTube’s director of product management, at last year’s Mobilize conference: