In this paper, we present a mobile robot global positioning system which combines passive RFID and monocular vision. The method is intended to solve the so-called kidnapped robot problem, that is, the problem of estimating the robot pose when it is first booted up or it has been kidnapped and taken to some unknown location. The main novelty of the approach is the use of fuzzy logic to cope with both uncertainty in RFID data and missing tag position information. Fuzzy logic allows us to manage inaccurate sensor data using rules easily understandable by humans. An additional advantage of the proposed method is that it does not require the robot to move around to solve ambiguities, nor does it need several tags to be properly arranged. Conversely, only one passive RFID tag and a single visual landmark are sufficient to univocally estimate the pose of the robot in the environment.