Amazon.com Inc. is letting its advertisers use external ad-buying tools for a portion of its inventory instead of going directly through the company.

The move extends Amazon’s effort to build its ad business and signals a degree of openness in an industry that has become wary of platforms that sell their own inventory and share limited data with advertisers, known in industry parlance as a walled garden.

Marketers now can purchase slots on Amazon’s Fire TV platform through advertising technology companies Dataxu Inc. or the Trade Desk Inc., in addition to working through Amazon itself, the company said in an online post.

The move comes as Amazon and other technology giants face regulatory scrutiny. The Justice Department is opening a broad antitrust review into whether dominant tech companies such as Amazon are unlawfully stifling competition.

Amazon has taken the No. 3 spot in the U.S. digital ad market, behind Facebook Inc. and Alphabet Inc.’s Google.

“These companies that are under fire from regulators around the world need to be seen as being as supportive of competition as possible,” said Brian Wieser, global president for business intelligence at WPP PLC’s media-agency conglomerate GroupM. “And having partnerships with independent players is helpful.”

“This shows that they side with the open internet,” Trade Desk Chief Executive Jeff Green said of Amazon. “Their being an open market that becomes friendly with the rest of the ecosystem helps them avoid the sort of draconian perspective that people have of Google and Facebook.”

Bringing in new buying channels also could help Amazon better compete with Roku Inc., which makes devices enabling consumers to stream video on their televisions. Roku sells inventory for third-party apps on its platform but also lets marketers buy video advertisements through Adobe Inc.’s advertising software.

TV platforms Fire TV and Roku stream digital video, a technology known as connected TV. About 195.1 million people have connected TVs in the U.S., according to data from research firm eMarketer Inc. That number is expected to grow to 201.7 million in 2020.

Write to Patience Haggin at patience.haggin@wsj.com