We analyze a novel antenna effect that resonantly enhances the photocurrent response of end-contacted P–i–N junction nanowire gratings, due to coupling of incident radiation into the grating's multiple-scattering electromagnetic modes. Quantitative characterization of these resonances was performed by spectral and polarization-resolved photocurrent measurements on gratings with N = 500, 200 and 100 nanowires, aided by electron beam-induced current measurements, and in excellent agreement with electromagnetic scattering theory. Despite the small scattering cross-section of each nanowire, with triangular cross-section (height 8 nm, width 6 nm), the measured quality factor of the resonances Q ≈ 10 exceeds that of the empty SiO2 cavity without degradation for gratings of as few as 100 nanowires. Photoresponse retains sinusoidal polarization anisotropy characteristic of single nanowires. We discuss strategies for improving Q and present a grating design tailoring resonant field enhancement at red, green or blue wavelengths, for three different grating periods of ℓ = 460, 400 and 320 nm.