A linear series-fed dual-port patch array working at 10 GHz with dual circular polarization or arbitrary linear polarization is proposed. This array consists of five circular patch elements which are serially excited by a traveling wave guided in a microstrip line. Despite that only five radiation elements are used, the array still exhibits high antenna efficiency greater than 93% averaged over the operation band. The typical trade-off between number of elements and antenna efficiency in series-fed array designs is overcome through the enhancement of coupling strength between the element and the feed line. The resulting high isolation between the two ports enables dual feeding configuration with low coupling. When only one port is excited, either right-handed or left-handed circular polarization radiation can be generated, as the sense of polarization is dependent on the traveling wave direction. When two ports are used simultaneously, arbitrary linear polarization can be realized by properly offsetting their phase difference. Based on reciprocity, this array can be also used for polarimetry applications. Experimental characterization on a fabricated antenna prototype validates the design.