Recently, an energy-harvesting antenna array has been used for modern wireless sensor applications. In this paper, the design of a two-port open-slot antenna array operated at the wireless local-area-network (WLAN) 2.4/5.2/5.8 GHz for wireless energy-harvesting sensor applications is investigated. The overall size of the proposed antenna is only 60 × 70 mm2 and printed on a flame retardant 4 (FR4) substrate with a thickness of 0.8 mm, a permittivity of 4.4, and a loss tangent of 0.0245 S/m. Furthermore, the antenna array has two ports and consists of two antenna arrays; the left side is designed for the WLAN 2.4 GHz band and the right side is designed for the WLAN 5.2/5.8 GHz bands. Both are printed on the front of the substrate, and the feeding network is printed on the back of the substrate. A metal reflector is added above the antenna to improve its radiation efficiency, gain, and directivity. The antenna of the proposed design can be operated at WLAN 2.4/5.2/5.8 GHz for wireless energy-harvesting sensor applications.