  We study the gravitational clustering of spherically symmetric overdensities
and the statistics of the resulting dark matter halos in the "symmetron model",
in which a new long range force is mediated by a $Z_2$ symmetric scalar field.
Depending on the initial radius of the overdensity, we identify two distinct
regimes: for small initial radii the symmetron mediated force affects the
spherical collapse at all redshifts; for initial radii larger than some
critical size this force vanishes before collapse because of the symmetron
screening mechanism. In both cases overdensities collapse earlier than in the
$\Lambda$CDM and statistically tend to form more massive dark matter halos.
Regarding the halo-mass function of these objects, we observe order one
departures from standard $\Lambda$CDM predictions. The formalism developed here
can be easily applied to other models where fifth-forces participate to the
dynamics of the gravitational collapse.
