More than 1.5 billion people watch YouTube Shorts every month, Alphabet Inc.’s Google disclosed on Wednesday, indicating the short-video service had reached a comparable scale to rival app TikTok after launching less than two years ago.

The figures provide a window into the hotly contested battle for supremacy in the short-video format, which is now central to how most people use social media.

YouTube is a pillar of Google’s business, generating more than $28 billion in advertising revenue last year, but it has of late been threatened by TikTok, owned by Beijing-based ByteDance Ltd., and competing services such as Meta Platform Inc.’s Instagram Reels.

Those rival services in particular have proved popular among advertisers hoping to reach younger audiences. Advertising revenue on YouTube rose by 14% in the first quarter compared with the same period last year, falling short of Wall Street’s expectations and marking a decline compared with the fourth quarter last year.

Google didn’t break out YouTube revenue separately until 2020, and investors at times have pressed the company to provide more insight into its performance.

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YouTube Shorts was first released in India in late 2020 and made available in the U.S. the next year. YouTube officials pitched the product to creators as part of a multiformat approach that includes longer videos—the service’s bread and butter—and newer outlets such as live streaming. Shorts are up to 60 seconds long.

“Our creators want fame or fortune or creative expression, or ideally all three,” said Tara Walpert Levy, vice president of Americas at YouTube. “This is the best way to deliver against that.”

Although apples-to-apples data aren’t publicly available, YouTube’s newly published viewership number would give Shorts a similar audience to TikTok and its Chinese counterpart Douyin, according to the most recent figures.

TikTok said in September the app had drawn more than 1 billion monthly active users. Third-party estimates have pegged the number as high as almost 1.6 billion at the end of March. ByteDance said in 2020 that Douyin had reached more than 600 million daily active users.

Meta, the parent company of Facebook and Instagram, hasn’t said how many monthly users interact with Reels. A company spokesperson pointed to an April earnings call during which Chief Executive Mark Zuckerberg

said Reels made up more than 20% of the time people spend on Instagram. Instagram passed 2 billion monthly active users last year, CNBC reported.

YouTube said its viewership statistic counts logged-in users who watched at least one video on the Shorts platform in April. It declined to share historical user data.

Rosanna Pansino, a cooking celebrity with 13.4 million subscribers on YouTube, said Shorts had recently become her top source for new viewers since she began making the videos six months ago. One Shorts video based on a joke about her diminutive height brought in 100,000 new subscribers, she said.

“The engagement is really high—I’d say higher than my regular length videos,” Ms. Pansino said. “That’s just incredible to me.”

Analysts have pointed to TikTok as a direct threat to YouTube, which has more than 2 billion monthly logged-in users across the platform.

Google said last month that it would begin automatically including Shorts in YouTube advertising plans and eventually allow companies to make their ads “shoppable.” Ms. Walpert Levy, the YouTube vice president, declined to comment on how it plans to share ad revenue with Shorts creators.

Mark Shmulik, internet analyst at Bernstein, said he viewed Shorts as a “defense product” to keep YouTube creators from devoting more energy to TikTok. Big question marks remain around how incumbents such as Google and Meta can make money off the short-video format, he said.

“It certainly feels like they’ve built compelling, competitive products,” Mr. Shmulik said. “I don’t know if I would call it a win yet.”