Netflix Inc. NFLX, -0.05% said its recent prices increases contributed to a disappointing number of new users in the third quarter, which comes as the online video-streaming company prepares to face a new challenge from a key rival.

Shares of the Los Gatos, Calif., company fell 26% to $331.25 in recent after-hours trading. Through Wednesday’s close, the stock had risen about 22% in 2014.

Netflix on Wednesday said it added 3 million streaming subscribers in the three months ended June 30, below the company’s forecast of 3.69 million.

In a letter to shareholders, Netflix cited its higher prices as well as the variables of its internal forecast.

“For the prior three quarters, we under-forecasted membership growth. This quarter we over-forecasted membership growth,” Netflix wrote. “We’ll continue to give you our internal forecast for the current quarter, and it will be high some of the time and low other times.”

Earlier this year, Netflix implemented a price increase, its first since 2011. New U.S. customers pay a $1 more a month, at $8.99, with existing customers grandfathered in for two years. New overseas customers saw a similar increase.

Netflix said the impact of the higher prices “appeared to be offset for about two months by the large positive reception to season two of [its series] ‘Orange is the New Black.”

Domestically, Netflix added 980,000 subscribers as well as 2.04 million users internationally. Both numbers were below the company’s forecast in July, when it projected adding 1.33 million domestic subscribers and 2.36 million overseas users.

“We remain happy with the price changes and growth in revenue and will continue to improve our service,” Netflix said. “The effect of slightly higher prices is factored into our Q4 forecast.”

For the fourth quarter, the company forecast per-share earnings of 44 cents, well below expectations of analysts polled by Thomson Reuters for 85 cents.

Also for the current quarter, the company said it expects to add a total of 4 million streaming subscribers, including 1.85 million net additions in the U.S. and 2.15 million internationally. A year earlier, Netflix added roughly 4.07 million subscribers overall.

An expanded version of this report appears at WSJ.com.