Abstract In this research, the effects of nanosilica and steel fibers on the impact resistance of ground granulated blast furnace slag based self-compacting alkali-activated concrete were investigated. Nanosilica volume fraction was kept constant at 2%. Two types of hooked-end steel fibers (Kemerix 30/40 and Dramix 60/80) and steel fiber volume contents (0.5% and 1%) were utilized to highlight the combined effects of nanosilica and steel fiber on the impact behavior. The fresh state and mechanical properties such as slump flow, L-box, V-funnel, compressive strength, modulus of elasticity, splitting tensile strength, and flexural strength were evaluated. The microstructure of the samples was examined using a scanning electron microscope. The impact resistance of the specimens was measured by a drop-weight test. Acceleration-time and force-time graphs were plotted and evaluated together with the crack photos of the specimens for the first and failure impactor drops. The incorporations of nanosilica and steel fiber improved splitting tensile strength, flexural strength, impact resistance, and energy absorption capacity, while they decreased compressive strength and modulus of elasticity. For the specimens without nanosilica and with 2% nanosilica, the impact energy improvements were five times and 12.5 times higher for 0.5% short fibrous, 20.5 times and 44.5 times higher for 1% short fibrous, 23.5 times and 31 times higher for 0.5% long fibrous, and 64 times and 144.5 times higher for 1% long fibrous specimens than the specimens without nanosilica and steel fiber, respectively. The long fibers were found more effective in mechanical strength and impact energy than short fibers, and the reinforcing efficiency of fibers enhanced with higher steel fiber volumes. The combined utilization of nanosilica and steel fibers have the potential to delay the crack formation and dissipate energy to the surrounding zones, and this potential increased with higher steel fiber lengths and volume ratios.