The company says the theft involved current postpaid customers.
The company's stock had dropped nearly 3% following the initial news.
Data was also stolen from about 850,000 prepaid customers and more than 40 million records of former or prospective customers, according to the company.
The breached data reportedly included customers' first and last names, date of birth, social security numbers, and driver's license information.
T-Mobile has suffered data breaches each of the past three years, impacting roughly 2 million customers in 2018, another million in 2019, and roughly 200,000 people in an attack at end of last year. The company's employee email system and some customer data were also hacked in March of 2020.
FOX Business' Breck Dumas contributed to this report.