Phase-of-arrival-based localization systems for passive ultrahigh frequency (UHF) radio-frequency identification (RFID) transponders must typically be calibrated to account for unknown phase offset values. Usually, reference measurements are performed using known tag positions. However, if the measurement scenario changes or other types of transponders are used, the system must be recalibrated. This paper introduces an automatic blind calibration method based on a Viterbi algorithm that requires no reference measurements. With this approach, the phase offset values are determined autonomously based on the measurement data set without any prior knowledge of the transponder positions, which are also estimated. To prove the calibration approach, a multichannel system demonstrator was used comprising a commercial off-the-shelf RFID reader, conventional passive UHF RFID tags, eight transceiver front ends, baseband hardware, and signal processing. Measurements were carried out in a 2-D indoor office environment, which was defined to be 3.5 m ×2.5 m. Compared with the common exhaustive calibration method using reference measurements at known tag positions, the automatic blind Viterbi-based approach offers similar results with a root-mean-square deviation of the phase offset values of 0.33 rad, which corresponds to an electrical length of 18 mm. Regarding the localization, the experiments showed accurate results with a root-mean-square error of 56 mm and a median absolute error of 21 mm.