Concepts and technologies are disclosed herein for out-of-band device verification of transactions. A processor can execute a verification service. The verification service can receive a verification request with data relating to a transaction between a computing device and an application. The verification service can identify an out-of-band device associated with the computing device and obtain verification data from the out-of-band device. The verification service can compare the verification data to data relating to the transaction to determine if the transaction is to be verified. Based upon the determination, the verification service can approve or block the transaction.