  The velocity field of dark matter and galaxies reflects the continued action
of gravity throughout cosmic history. We show that the low-order moments of the
pairwise velocity distribution, $v_{12}$, are a powerful diagnostic of the laws
of gravity on cosmological scales. In particular, the projected line-of-sight
galaxy pairwise velocity dispersion, $\sigma_{12}(r)$, is very sensitive to the
presence of modified gravity. Using a set of high-resolution N-body simulations
we compute the pairwise velocity distribution and its projected line-of-sight
dispersion for a class of modified gravity theories: the chameleon \fR gravity
and Galileon gravity (cubic and quartic). The velocities of dark matter halos
with a wide range of masses would exhibit deviations from General Relativity at
the $(5-10)\sigma$ level. We examine strategies for detecting these deviations
in galaxy redshift and peculiar velocity surveys. If detected, this signature
would be a "smoking gun" for modified gravity.
