The Warner Bros. movie studio will make 10 films exclusively for its sister streaming service HBO Max to premiere in 2022, said WarnerMedia Chief Executive Jason Kilar.

The move comes as HBO and its enhanced streaming platform, HBO Max, prepare to lose access to films and TV shows from rival studios that are now keeping more of their content in-house for their own streaming services.

Earlier this month, Comcast Corp.’s Universal Studios—which has an exclusive deal with HBO to carry its movies after their theatrical release—said that in the future its movies would go to NBCUniversal’s own Peacock streaming service within the first four months of hitting theaters.

Universal subsequently struck a deal giving Amazon.com Inc.’s streaming platforms access to its movies for 10 months after their four-month window on Peacock passes, an option Mr. Kilar said HBO wasn’t interested in.

Such deals for nonexclusive rights aren’t strategic for HBO and HBO Max, he said in an interview Thursday, adding that “us investing in our own organization is our best use of resources.”

Walt Disney Co. is expected to keep movies from its Fox movie studio for its Disney+ and Hulu platforms when its HBO pact expires after 2022 as well.

WarnerMedia parent AT&T Inc. said in its second-quarter earnings announcement Thursday that HBO and HBO Max together have 47 million domestic subscribers, an increase of nearly three million from three months earlier.

Mr. Kilar said the subscriber growth is coming primarily from new customers to HBO Max as opposed to existing HBO subscribers activating the Max platform. He said he expects that trend to continue and is the reason why the average revenue per user for the HBO unit has grown significantly over the past year.

Mr. Kilar also dismissed concerns that AT&T—which agreed to spin off WarnerMedia and combine that unit with Discovery Inc. in May—isn’t fully committed to the heavy spending on content he has planned for HBO Max.

“You can see this in our numbers. We are investing aggressively in our storytelling,” he said, adding, “I feel very good about where we are now and how we are setting ourselves up for 2022 and 2023.”

Mr. Kilar has said he would stay in his role at least through the completion of the merger, which will result in a new, publicly traded entertainment behemoth led by Discovery CEO David Zaslav. Mr. Kilar said in the interview that he is focusing on expanding HBO Max around the globe and doesn’t know what will happen a year from now, when the Discovery deal is expected to close pending regulatory approval.

One of the factors driving HBO Max’s growth is WarnerMedia’s decision to put all of the 2021 Warner Bros. films on the streaming platform simultaneously with their theatrical release—a move driven by the Covid-19 pandemic’s impact on the traditional movie business as theaters shut down for months last year. Mr. Kilar reiterated that this strategy is still meant to apply for one year only. Next year, most Warner Bros. movies will premiere in theaters only and then appear on HBO Max 45 days later, a much shorter window than in previous years.

Earlier this week, WarnerMedia unveiled long-anticipated plans to roll out a direct-to-consumer streaming service for its CNN cable news channel. The platform has been dubbed CNN+ and is slated to launch in the first quarter of 2022. Mr. Kilar said a price tag for the service has yet to be determined.

Write to Joe Flint at joe.flint@wsj.com