T-Mobile said it was aware of the claims made by a hacker in a forum post and has been "actively investigating their validity."
"We do not have any additional information to share at this time," the company added in a statement.
The stock fell 3.04% on Monday to $140.51.
Vice Media's Motherboard was the first to report on the possible data breach.
The forum post itself doesn't mention T-Mobile, but the seller told Motherboard data has been obtained related to more than 100 million people, and that the data came from T-Mobile servers. T-Mobile has nearly 104.8 million U.S. customers.
The data include social security numbers, phone numbers, names, physical addresses, unique IMEI numbers, and driver's license information, the seller said.
Motherboard reported that it has seen samples of the data, and confirmed they contained accurate information on T-Mobile customers.
The seller has been asking for 6 bitcoin, around $270,000, for a subset of the data containing 30 million social security numbers and driver's licenses. The seller said they were privately selling the rest of the data at the moment, Motherboard reported.
Electronic Arts (EA) - Get Free Report reportedly suffered a massive data breach earlier this summer that could have resulted in the loss of source codes for some if its key products.
Vice reported that hackers claimed access to course codes for "FIFA 21," one of Electronic Arts' principal game offerings, as well as the code for its matchmaking server.