We report the observation of an in-plane nodal loop accompanying highly dispersive Dirac fermions with a velocity of $1.2\times {10}^6$ m/s in a superconductor ${\text{ZrP}}_{1.24}{\text{Se}}_{0.57}$ by employing angle-resolved photoemission spectroscopy. This nodal loop arises in principle from the P square net as long as nonsymmorphic symmetry is preserved. We have also found that two quasisquare Fermi surfaces that branch from the nodal loop at $-1.2$ eV are largely parallel in favor of nesting, and that two additional electron pockets are present at the Brillouin-zone center unlike in the case of ZrSiS. Our findings demonstrate that a P square net embedded in a superconductor provides a new playground for emergent exotic phenomena coexisting with superconductivity.

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• Dirac fermions
• Electronic structure
• Dirac semimetal
• Superconductors
• Angle-resolved photoemission spectroscopy
• First-principles calculations
• Tight-binding model