Materials with graphenelike layers attract tremendous attention due to their electronic structures and superconducting properties. In this study, we synthesized $\mathrm{La}{\mathrm{P}}_2$ polycrystalline and observed a superconducting transition at around 30 GPa. The critical temperature $T_c$ increases monotonically with pressure, which is nearing saturation and reaches 7.8 K at 78 GPa. The synchrotron x-ray diffraction experiments confirm the superconducting transition originates from a structure transition to the $P6\text{/}mmm$ phase under high pressure, suggesting the observation of graphenelike phosphorus layers in transition-metal phosphides. By first-principles calculations, we provide more evidence for the stability of the graphenelike phosphorus layers in $\mathrm{La}{\mathrm{P}}_2$. Our findings are helpful for the understanding of the $\mathrm{La}{\mathrm{P}}_2$ phase diagram under high pressure and could shed light on the research of unique structures in transition-metal phosphides under high pressure.

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• High-pressure studies
• Phase transitions
• Superconductivity
• First-principles calculations