  Single crystal and polycrystalline samples of MoP$_4$ with a
black-phosphorus-derived structure have been successfully synthesized by a
high-pressure technique. The polycrystalline samples show a large positive
magnetoresistance and a small negative Seebeck coefficient at low temperatures,
reflecting a semi-metallic nature with high-mobility electrons. Consistent with
the transport properties, the band structure calculation reveals a
semi-metallic state with the presence of two types of Dirac nodes slightly
below the Fermi level. The Dirac node along the $\Gamma$-X direction normal to
the phosphorus layers is gapped out in the presence of spin-orbit coupling
(SOC), whereas the band crossing at the Z-point is immune to SOC because of the
non-symmorphic symmetry. This work demonstrates a great potential of
phosphorus-based layered Zintl compounds for topological semimetal candidates
allowing chemical band engineering.
