Examples described herein include systems, methods, instructions, and other implementations for data security with integrated installment payment systems. In one example, account security system receives a checkout communication that includes data describing a validated checkout system of a merchant system. A client token is transmitted in response to an authentication that the checkout communication is from the validated checkout system, and an account communication including the client token and secure client information is received from a client device. An installment payment communication associated with the secure transaction is received from a system other than a merchant system involved in the transaction. The secure transaction is then facilitated following receipt of the installment payment communication.