The application of UHF radio-frequency identification (RFID) systems is rapidly increasing. However, the reliability of indoor telecommunication is adversely affected by multiple reflection interference. Consequently, to establish a highly reliable UHF-RFID system, it is essential to improve the multipath environment. This paper describes an ultrathin microwave absorber that is designed to improve the multipath environment of UHF-RFID systems. This absorber is optically transparent so that it does not create blind areas for surveillance cameras installed near an UHF-RFID system (e.g., at the entrance of an office or a warehouse). The improved tag identification probability using this absorber is demonstrated in an actual warehouse.