  We report spin-split Landau levels of quasi-two-dimensional Dirac fermions in
a layered antiferromagnet EuMnBi$_2$, as revealed by interlayer resistivity
measurements in a tilted magnetic field up to $\sim$35 T. The amplitude of
Shubnikov-de Haas (SdH) oscillation in interlayer resistivity is strongly
modulated by changing the tilt angle of the field, i.e., the
Zeeman-to-cyclotron energy ratio. The effective $g$ factor estimated from the
tilt angle, where the SdH oscillation exhibits a phase inversion, differs by
approximately 50% between two antiferromagnetic phases. This observation
signifies a marked impact of the magnetic order of Eu sublattice on the
Dirac-like band structure. The origin may be sought in strong exchange coupling
with the local Eu moments, as verified by the first-principles calculation.
