  We study the Sivers single spin asymmetry of the Drell-Yan lepton pair
production in the small-$x$ regime. We find that in the corresponding kinematic
region the spin asymmetry calculated in the small-$x$ approach is consistent
with either the usual transverse-momentum-dependent factorization formalism or
the collinear factorization formalism, respectively. We estimate the Sivers
asymmetry for both polarized p+p and p+A collisions and argue that the
Drell-Yan production is an interesting and unique probe for both the transverse
spin physics and the small-$x$ saturation effect.
