A modular transaction card assembly includes a card frame having the traditional dimensions of a credit card, and a transaction card that is smaller than a traditional card and that fits into a receptacle of the card frame. Each of the card frame and the transaction card may be capable of performing contactless data transactions individually. The combined assembly of the card frame with the transaction card secured in the receptacle is also capable of performing contact data transactions, and in some instances, with an identifier that is distinct from the card frame or the transaction card alone. The card frame may include a processor enabling it to perform data encryption and authentication of the smaller transaction card.