Roku Inc. ROKU -3.35% said it is launching a marketplace where TV networks can sell commercial space that targets specific audiences, adding to an array of companies trying to make TV advertising more targeted.

21st Century Fox, FOX 1.54% AT&T’s WarnerMedia and Viacom Inc. have signed on to sell TV ad inventory in Roku’s “Audience Marketplace,” where they will be able to use its viewing data to help advertisers target specific customers.

The ads would appear in programming people see when they watch TV through their Roku TVs or streaming devices.

“Over-the-top distribution has been a key audience driver for Turner’s portfolio of premium content, with Roku being one of the preeminent partner platforms,” said Larry Allen, vice president of ad innovation and programmatic solutions at Turner, in a statement. “Participating in Roku’s Audience Marketplace gives us access to rich insights and enhanced audience targeting capabilities.”

Roku, which sells devices that stream TV programming, has growing ambitions in the advertising business. The company reported $75 million in first quarter revenue from its platform business, which includes advertising and content services like licensing and accounts for 55% of the company’s revenue.

Roku’s initiative is one of several aimed at updating the antiquated TV ad business. Fox, Turner, Viacom and NBCU recently joined forces to create a separate consortium to help advertisers figure out which shows are likely to reach specific audiences. AT&T Inc. also has considered creating a marketplace for TV and digital ad inventory.

It’s unclear how much commercial space the media companies will be willing to sell in the Roku marketplace.

TV networks typically allocate two minutes of ad time per hour to TV distributors, who then sell the space to local advertisers. The TV networks then sell national ads aimed at broad groups of people in their portion of the programming.

If the networks want to use Roku’s system to allocate more commercial time for targeting precise categories of viewers -- say, people in the market for a car in certain zip-codes -- there might be risks. They might earn a higher price per ad, but they could earn more overall from ads with national reach.

“What the Audience Marketplace does is provide our publisher partners with a path to improved monetization across their inventory on Roku,” said a Roku spokeswoman. “Some of their inventory may already be accounted for as part of their upfront sales, but the Roku Audience Marketplace gives them the flexibility to enhance all their Roku inventory and at a premium price.”

Advertisers can take advantage of the Audience Marketplace through an automated capability or traditional direct-selling methods, according to Roku.

Write to Alexandra Bruell at alexandra.bruell@wsj.com