Coupling of diffracted waves in gratings with the waveguide modes gives rise to the guided mode resonances (GMRs). The GMRs provide designer linewidth and resonance intensity amidst a broad background, and thus have been widely used for numerous applications in visible and infrared spectral regions. Here, terahertz GMRs are demonstrated in low-loss, all-dielectric metasurfaces, which are periodic square lattices of silicon cuboids on quartz substrates. The silicon cuboid lattice simultaneously acts as a diffraction grating and an in-plane slab waveguide, thereby resulting in the formation of terahertz GMRs. At oblique incidence, two distinct frequency detuned GMRs are observed. The frequency difference between these two GMRs increases at larger angle of incidence. However, extremely small angle of incidence causes destructive interference between these counter-propagating GMRs that leads to a nonradiative symmetry-protected bound state in the continuum. GMRs in all-dielectric silicon metasurfaces can have potential applications in the realization of efficient terahertz devices such as high-Q transmission filters with angular spectral selectivity, ultrafast modulators, and free-space couplers. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim