  We report two-dimensional quantum transport in SrMnBi$_2$ single crystals.
The linear energy dispersion leads to the unusual nonsaturated linear
magnetoresistance since all Dirac fermions occupy the lowest Landau level in
the quantum limit. The transverse magnetoresistance exhibits a crossover at a
critical field $B^*$ from semiclassical weak-field $B^2$ dependence to the
high-field linear-field dependence. With increase in the temperature, the
critical field $B^*$ increases and the temperature dependence of $B^*$
satisfies quadratic behavior which is attributed to the Landau level splitting
of the linear energy dispersion. The effective magnetoresistant mobility
$\mu_{MR}\sim 3400$ cm$^2$/Vs is derived. Angular dependent magnetoresistance
and quantum oscillations suggest dominant two-dimensional (2D) Fermi surfaces.
Our results illustrate the dominant 2D Dirac fermion states in SrMnBi$_2$ and
imply that bulk crystals with Bi square nets can be used to study low
dimensional electronic transport commonly found in 2D materials like graphene.
