Roku Inc. reported a slowdown in new active accounts for its streaming services in the latest quarter and guided for lower-than-expected revenue for the holiday quarter, as global supply chain issues have affected new TV sales.

Roku added 1.3 million active accounts in the third quarter, bringing its total active users to 56.4 million. Analysts polled by FactSet were expecting roughly 1.7 million additional accounts.

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Roku Inc. reported a slowdown in new active accounts for its streaming services in the latest quarter and guided for lower-than-expected revenue for the holiday quarter, as global supply chain issues have affected new TV sales.

Roku added 1.3 million active accounts in the third quarter, bringing its total active users to 56.4 million. Analysts polled by FactSet were expecting roughly 1.7 million additional accounts.

Shares of Roku fell by about 8% in after-hours trading. Shares closed Wednesday at $313.66 and are down 5.5% so far this year.

“We believe that the slowdown in active account growth rate this quarter was, in large part, attributable to global supply chain disruptions that have impacted the U.S. TV market,” Chief Executive Anthony Wood and finance chief Steve Louden said in a letter to shareholders. “Specifically, overall U.S. TV sales in Q3 fell below pre-Covid 2019 levels.”

Roku sells smart televisions with built-in streaming technology, as well as devices that users can plug into TVs. The company generates the bulk of its revenue by selling ads on streaming channels and taking a share of the streaming services’ subscription revenue and ad inventory.

For the fourth quarter, the company guided for total net revenue ranging from $885 million to $900 million. Analysts were expecting $946 million.

In the third quarter, Roku reported net profit of $68.9 million, or 48 cents a share, up more than fivefold from $12.9 million, or 9 cents a share, a year earlier. Revenue rose 51% to $680 million, falling short of analyst estimates of $683.5 million.

On a call to discuss the company’s earnings, Mr. Louden said the global supply chain disruptions would likely affect the overall holiday season in terms of shipping delays, product-availability issues and product-price increases. “We expect the U.S. TV market to continue to be significantly impacted by these issues,” he said.

Roku was shielded from the effects of Apple’s new privacy policy, which affected the ad businesses of companies like Meta Inc.—as Facebook recently rebranded itself—and Snap Inc. The hit to those ad businesses actually benefited Roku, said Scott Rosenberg,

Roku’s senior vice president and general manager of media business, since it prompted advertisers to spend more on streaming video ads.

The company said its average revenue per user surpassed $40 for the first time, growing 49% from a year earlier.

Mr. Rosenberg declined to share any information about Roku’s continuing carriage dispute with Alphabet Inc. -owned video-streaming platform YouTube. YouTube removed its YouTube TV app, which lets subscribers stream dozens of live-TV channels for a monthly fee, from Roku on April 30 after the two parties couldn’t agree to a new contract.

Roku’s deal to carry the more popular free YouTube app expires on Dec. 9.

Write to Patience Haggin at patience.haggin@wsj.com and Robert Barba at Robert.Barba@wsj.com