Pressure-induced superconductivity in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>La</mml:mi> <mml:msub> <mml:mi mathvariant="normal">P</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:math> with a grapheneli

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.