Boise Police detectives tell KBOI-TV that a customer spotted the device Friday evening at the U.S. Bank on 17th and State Street.
Skimmers are devices that attach to existing card readers that steal your information off the magnetic strip and also record your PIN number when you enter it into the keypad, which can lead to the customer's bank account information.
"We are working with customers who may have used the ATM during the time that the skimmer was on the machine...," a U.S. Bank spokesperson said.
There's no word yet on whether any suspects have been identified and it's also unclear how many customers may have used the ATM while the skimmer was attached.
Earlier this March, two skimmer devices were discovered at a gas station along Interstate-84 and Broadway.
The skimmers in those instances were turned over to the U.S. Secret Service.