Radio frequency identification (RFID) is widely used as a tagging system to facilitate physical transactions in the real world. Thanks to the availability of inexpensive passive RF tags, RFID technology is now the catalyst of Internet of Things, i.e., every object can be uniquely identified in an Internet-like way. In the future, many individual RFID systems are likely to be integrated into a few exascale RFID systems. In an integrated RFID system, service providers (SPs) that offer RFID-based data service and clients that use the data service are different organizations. As a consequence, quality of data in terms of authenticity is of significant concern. In this paper, we first formulate a data verification problem in RFID systems and build a model of integrated RFID systems where multiple SPs and clients exist. Then, we propose two data verification protocols to ensure data generated by SPs associated with a particular tag and has not been modified. In addition, we implement our system as a prototype. The computer simulations, analyses, and testbeds based on the prototype all demonstrate that the proposed verifiable integrated RFID system achieves a high level of security and performance.