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We report an experimental observation of radiation-induced optical absorption in undoped congruent lithium niobate (LiNbO₃) crystals between the 190 nm and 3200 nm wavelength range. It was found that high-dose (up to 288 Gy) gamma rays did not significantly affect the optical absorption of LiNbO₃ during that wavelength range. However, upon the order of 10¹⁶ cm⁻² neutron irradiation, the change in the absorption coefficient was up to 0.6 cm⁻¹ at a wavelength of 532 nm and remained after 13 days in the dark at room temperature. The nonlinear optical performance was characterized by conducting third-order quasi-phase-matched second harmonic generation in neutron-irradiated periodically-poled LiNbO₃, showing that the phase-matching condition was altered, and the conversion efficiency was still comparable with a non-irradiated one.