In this paper, reconfigurable circularly-polarized (CP) cavity-backed slot antenna (CBSA) and antenna array with frequency and polarization agility are proposed. Two degenerate modes TE101 and TE011 are respectively perturbed by the metal posts along y-axis (named as MP-1) and x-axis (named as MP-2), and independent frequency tuning is obtained. Then, two crossed slots are placed on the top wall to radiate the electromagnetic energy of TE101 and TE011 modes, respectively. In this context, circular polarization can be realized by setting different lengths of MP-1 and MP-2. Besides, reconfigurable polarization between left-hand and right-hand circular polarization can be realized by exchanging the lengths of MP-1 and MP-2. Meanwhile, reconfigurable operating frequency can be realized by simultaneously increasing or decreasing the lengths of MP-1 and MP-2. In addition, a reconfigurable 4×4 CP array without introducing power dividers and without increasing the number of tuning metal posts is further investigated. Finally, the reconfigurable 4×4 CP array is fabricated and tested. Reconfigurable frequency, reconfigurable polarization, high total efficiency, and stable radiation pattern are simultaneously obtained. Good agreement between measured and simulated results well validates the proposed design concept.