A proximity card or prox card also known as a key card or keycard is a contactless smart card which can be read without inserting it into a reader device, as required by earlier magnetic stripe cards such as credit cards and contact type smart cards. The proximity cards are part of the contactless card technologies. Held near an electronic reader for a moment they enable the identification of an encoded number. The reader usually produces a beep or other sound to indicate the card has been read.

The term "proximity card" refers to the older 125 kHz devices as distinct from the newer 13.56 MHz contactless smartcards. Second generation prox cards are used for mass and distance reading applications. Proximity cards typically have a read range of up to 50 cm (20 in) which is the main difference from the contactless smartcard with a range of 2 to 10 cm (1 to 4 in). The card can often be left in a wallet or purse, and read by simply holding the wallet or purse near the reader. These early proximity cards can't hold more data than a magnetic stripe card, and only cards with smart chips (ie, contactless smartcards) can hold other types of data like electronic funds balance for contactless payment systems, history data for time and attendance or biometric templates. When used without encoding data, only with the card serial number, contactless smartcards have similar functionalities to proximity cards.