Mechanoluminescent materials, which are capable of converting some ignored mechanical stimuli like friction, pressure, and distortion into useful photons, are gathering increasing attention nowadays, especially considering that a new round of energy crisis is sweeping across the globe. However, the wavelength of mechanoluminescence (ML) reported so far overlaps seriously with the solar spectrum, limiting the practical applications of mechanoluminescent materials to some extent. Here, we report the ultraviolet-C (UVC) ML from NaYF4:Pr3+ with the central wavelength at ∼250 nm, which is one of the shortest wavelengths for ML and is ascribed to the 1S0 → 3F4 transition of Pr3+. After being charged by x rays, NaYF4:Pr3+ is able to emit ML under the stimulation of external stress. The presence of UVC ML has been confirmed by both a spectrometer and a solar-blind camera. We confirm that the UVC ML belongs to the trap-controlled ML via thermoluminescence and related trap-filling-emptying measurements. With the aid of the density functional theory, we discuss the possible mechanism of the UVC ML from NaYF4:Pr3+