Silicon nanowire (SiNW) solar cells are attractive candidates as bottom cell material for III-V-based multijunction solar cells (MJSCs) used in space and one-sun terrestrial applications. This is because they are lightweight and have better light trapping and shorter charge separation distance in the junction than their planar counterparts. In this work, we characterized SiNW single junction solar cell (1JSC) with commercial lattice-matched (LM) InGaP/GaAs dual junction solar cell (2JSC). The external quantum efficiency (EQE) of SiNW 1JSC with and without InGaP/GaAs 2J filter and each absorber of the InGaP/GaAs 2JSC were measured. As shown in Fig. 1, adding a SiNW as a bottom cell to the III-V 2JSC extended the absorption range to the infrared region, from about 850 nm to 1200 nm. We also obtained the current-voltage (J-V) characteristics curves from SiNW 1JSC with and without InGaP/GaAs 2J filter, InGaP/GaAs 2JSC, and series-connected InGaP/GaAs 2JSC and SiNW 1JSC to emulate a three-terminal, triple-junction solar cell, hereby referred to as InGaP/GaAs//SiNW 3JSC. The series-connected InGaP/GaAs//SiNW 3JSC was measured using 2 independent, global air mass 1.5G standard (AM 1.5G) 1-sun calibrated solar simulators. The resultant J-V curves and the derived electrical performances are shown in Fig. 2 and Table 1, respectively. Without NW passivation and H₂ annealing, adding a SiNW to the III-V 2JSC provided a 0.77% efficiency increase. Thus, to achieve the same quality of the planar Si-based III-V MJSCs that can provide more than 30% conversion efficiency, reduction of surface defects by passivation and H₂ annealing are necessary steps in SiNW fabrication.