  We study the effect of peculiar motion in weak gravitational lensing. We
derive a fully relativistic formula for the cosmic shear and the convergence in
a perturbed Friedmann Universe. We find a new contribution related to galaxies
peculiar velocity. This contribution does not affect cosmic shear in a
measurable way, since it is of second order in the velocity. However, its
effect on the convergence (and consequently on the magnification, which is a
measurable quantity) is important, especially for redshifts z < 1. As a
consequence, peculiar motion modifies also the relation between the shear and
the convergence.
