The plastic deformation behavior of single crystals of α-Nb₅Si₃ with the tetragonal D8l structure has been investigated by micropillar compression at room temperature as a function of crystal orientation and specimen size. Three slip systems, (001)<010>, {110}<110> and {011}<111>, are found to be operative in micropillar specimens of α-Nb₅Si₃ single crystals at room temperature, as in the case of isostructural Mo₅SiB₂. The CRSS values obtained for the three slip systems are extremely high above 2.0 GPa and exhibit the ‘smaller is stronger’ trend, which can be approximated by the inverse power-law relationship. The fracture toughness evaluated by single-cantilever bend testing of a chevron-notched micro-beam specimen is 1.79 MPa m^1/2, which is considerably lower than that (2.43 MPa m^1/2) reported for isostructural Mo₅SiB₂. The selection for the dissociation schemes and possible glide planes for dislocations of the three slip systems is discussed based on generalized stacking fault energy (GSFE) curves theoretically calculated by first-principles calculations.