A Personal Unblocking Code (PUC) or Personal Unblocking Key (PUK) is used in GSM mobile phones and some smartcards to unblock a blocked card.

Most mobile telephones offer the feature of personal identification number (PIN) protection. After switching on the phone, if the PIN security function is not switched off, the user is requested to enter a 4-8 digit PIN enabling the phone's non-emergency calling functions. If the wrong PIN is typed in more than three times, either the SIM card, the device or both become locked. They can be reverted to their original unlocked state by entering a PUC, provided by the service operator through verification. If the wrong PUC is entered ten times in a row, the device will become permanently blocked and unrecoverable, requiring a new SIM card. Cellular phone users are therefore advised by most providers to keep their PUC written down in a safe place separate from the device.