A dual-band circularly polarized (CP) tag antenna is presented in this paper, which combines ultrahigh-frequency (UHF) and ultrawideband (UWB) operations for radio-frequency identification (RFID) and high-precision localization applications. The proposed antenna consists of two CP radiating elements with separated ports, where the UHF element is a planar loop antenna loaded with two pairs of meander lines of different dimensions for compactness and CP radiation, while the UWB element is a modified Archimedean spiral antenna with a tapered metallization ratio and encircled by the UHF loop. In addition, a miniaturized shunt-stub is employed in the UHF element to realize conjugate matching between the antenna and the UHF RFID chip. Experimental characterization of a fabricated prototype has validated the design, which shows that the proposed antenna exhibits very good radiation performance at both the bands, with −10 dB power reflection bandwidths of 0.86–0.975 GHz and 2.6–12 GHz, and 3 dB axial-ratio bandwidths of 0.911–0.938 GHz and 3.1–11.1 GHz, respectively. This antenna can find applications in RFID and indoor positioning systems.