  We present a four-dimensional (4D) ${\cal N}=1$ superfield description of
supersymmetric Yang-Mills (SYM) theory in ten-dimensional (10D) spacetime with
certain magnetic fluxes in compactified extra dimensions preserving partial
${\cal N}=1$ supersymmetry out of full ${\cal N}=4$. We derive a 4D effective
action in ${\cal N}=1$ superspace directly from the 10D superfield action via
dimensional reduction, and identify its dependence on dilaton and geometric
moduli superfields. A concrete model for three generations of quark and lepton
superfields are also shown. Our formulation would be useful for building
various phenomenological models based on magnetized SYM theories or D-branes.
