This paper presents a novel circularly polarized (CP) planar aperture antenna for cost-sensitive millimeter-wave, particularly fifth generation (5G), applications. The proposed antenna, which is a differential one, has high gain and wide bandwidth and can be built on a single-layer laminate using standard printed-circuit board (PCB) technology. This makes the antenna both low cost and easy to integrate with common differential circuits. The antenna consists of an opening cavity formed by metalized vias, which encloses a set of loop-like strips that are fed differentially. The strips carry traveling wave current which together with the aperture's E-fields distribution give rise to CP radiation. The aperture size of the antenna can be extended for higher gain by adopting a dual- or quadruple-loop-like strips configuration enclosed by a larger opening cavity. The fabricated quadruple-loop-like strips-based antenna prototype designed to operate at 60-GHz band could achieve 3-dB axial ratio (AR) bandwidth of 16.7% (56-66.2 GHz) within its -10-dB impedance bandwidth and stable right-handed CP (RHCP) gain throughout the operating bandwidth with a peak of 14.6 dBi.