Chipotle restaurant officials told investors they detected "unauthorized activity" on a payment processing network.
The company believes it has taken the right steps to stop the activity, but this could have affected people who visited the restaurant chain from March 24 to April 18.
Company officials haven't revealed any more information about the breach.
The breach is overshadowing the company's latest success story, which includes better-than-expected earnings as more customers head to Chipotle restaurants.
Analysts don't seem to think this will hurt Chipotle too much, though. They expect the restaurant to continue making gains throughout the rest of the year.