MyFitnessPal is a popular health and fitness iOS app and website that's used by many iPhone and iPad owners, and it is consistently among the top Health and Fitness apps in the App Store.
The breach, which happened in late February, included usernames, emails, and encrypted passwords. Credit card numbers from paying customers were not accessed.
Under Armor has already been sending emails and in-app messages to its customers who were affected by the breach. The company says it is working with "leading data security firms" to assist in its investigation.
MyFitnessPal users will be required to change their passwords.