The young $sigma$-Orionis cluster is an important location for understanding the formation and evolution of stars, brown dwarfs, and planetary-mass objects. Its metallicity, although being a fundamental parameter, has not been well determined yet. We present the first determination of the metallicity of nine young late-type stars in $sigma$-Orionis. Using the optical and near-infrared broadband photometry available in the literature we derive the effective temperatures for these nine cluster stars, which lie in the interval 4300--6500 K (1--3 Msuno). These parameters are employed to compute a grid of synthetic spectra based on the code MOOG and Kurucz model atmospheres. We employ a $chi^2$-minimization procedure to derive the stellar surface gravity and atmospheric abundances of Al, Ca, Si, Fe, Ni and Li, using multi-object optical spectroscopy taken with WYFFOS+AF2 at at the William Herschel Telescope ($lambda/deltalambdasim7500$). The average metallicity of the $sigma$-Orionis cluster is [Fe/H] $ = -0.02pm0.09pm0.13$ (random and systematic errors). The abundances of the other elements, except lithium, seem to be consistent with solar values. Lithium abundances are in agreement with the cosmic $^7$Li abundance, except for two stars which show a $log epsilon(mathrm{Li})$ in the range 3.6--3.7 (although almost consistent within the error bars). There are also other two stars with $log epsilon(mathrm{Li})sim 2.75$. We derived an average radial velocity of the $sigma$-Orionis cluster of $28pm4$km/s. The $sigma$-Orionis metallicity is roughly solar.
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