The company gave the warning as it announced record results, driven by the popularity of the iPhone. Sales of the handset have grown for the first time in a year and come alongside interest in accessories like the AirPods.
But as Tim Cook announced those results, he said the company is concerned about the virus outbreak in China, which is both a major major market and home for large amounts of its manufacturing business.
Mr Cook indicated that Apple had given a wider prediction for its next quarter than usual because it was unclear how the virus outbreak would hit its business.
"We have limited travel to business-critical situations as of last week," he said. "The situation is emerging, and we're still gathering lots of data points and monitoring it very closely."
Apple has suppliers in the Wuhan area, the heart of the outbreak, but has alternatives, Cook said. Factories outside Wuhan area will not reopen after the Lunar New Year holiday until 10 February, Mr Cook said, but Apple built the delayed restart into its wider revenue forecast.
Apple has shut one store in China, he said, and reduced hours at others because of lower foot traffic, Cook said. Third-party stores that sell Apple products are also facing some closures, Cook said.
Apple is "forecasting a stronger Q2 than analysts predicted, but the fact that the coronavirus is spreading in unpredictable ways in China, where Apple has most of its hardware built, could upset this optimistic forecast," said eMarketer principal analyst Yoram Wurmser.
Additional reporting by Reuters