  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
($\lambda/\delta\lambda\sim7500$). The average metallicity of the
$\sigma$-Orionis cluster is [Fe/H] $ = -0.02\pm0.09\pm0.13$ (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 $28\pm4$km/s. The $\sigma$-Orionis metallicity is
roughly solar.
