We present the main parameters, design features, and optical characterization of the GrAnada LIghtning Ultrafast Spectrograph (GALIUS): a portable, ground-based spectrographic system intended for analysis of the spectroscopic signature of lightning. It has been designed to measure the spectra of the light emitted from natural and triggered lightning and artificial electrostatic discharges at recording speeds up to 2.1 Mfps. It includes a set of four interchangeable grisms covering different spectral ranges (from 375 nm to 854.5 nm) with spectral resolutions from 0.29 nm to 0.76 nm. A set of 10 collector lenses allows the recording of the spectrum of electrostatic discharges and lightning in different scenarios.