The videos will play automatically for a small percentage of people who use Twitter's iOS app in the U.S. "We're running a small test on a few variations on the video playback experience," a Twitter spokesman said.
The test, first reported by Advertising Age, applies both to videos uploaded by users and to those posted by advertisers, but it doesn't apply to looped videos from Twitter's Vine service.
It's not clear how long the test will last. It's designed to let Twitter see how receptive users are to having videos play automatically, as it considers rolling the feature out more broadly and charging advertisers for it. For existing video ads, Twitter charges advertisers only when a user clicks on a video.
Autoplaying videos are more attractive to marketers because they get ads in front of more eyeballs, even if they're more expensive to run.
On YouTube, videos have to be clicked on before any ads run, but Facebook began rolling out autoplaying videos in 2013. Since then, more videos are being posted on Facebook, and the amount of video in people's feeds more than tripled in 2014.
Twitter's autoplaying videos will initially play without sound. If a mobile user taps on the ad, it will expand to full screen and play with sound.
For the test, roughly 2 percent of Twitter users will see the autoplaying ads. With about 288 million people logging in each month, that's more than 5 million people.
Zach Miners covers social networking, search and general technology news for IDG News Service. Follow Zach on Twitter at @zachminers. Zach's e-mail address is zach_miners@idg.com