A novel design concept of bandwidth-enhanced full-metal circularly-polarized (CP) cavity-backed slot antenna and array (CBSA) based on two pairs of degenerate modes in a single cavity is proposed in this paper. Conventional CP antenna based on one pair of degenerate modes only has one axial-ratio (AR) minima and causes a narrow AR bandwidth. Two pairs of degenerate modes in a proposed multimode cavity resonator can initially produce two separate CP operating bands. Then, a pair of metal prisms is introduced to allocate the two bands in proximity to each other, thus obtaining a wide operating bandwidth with two AR minima. As the radiation slots are directly fed by the electric field of the cavity modes, 2×2 cross-slots are placed on the top wall so as to comprise a CP array. As such, a very simple wideband CP antenna array is realized by virtue of a single multiple-mode resonator without needing extra power dividers. Finally, the 2×2 wideband CP antenna array is fabricated and measured. Measurement shows that the proposed antenna array can achieve 15% bandwidth, a peak gain of 11.8 dBic, and a peak total efficiency of 96%.