Pressure-tuned one- to quasi-two-dimensional structural phase transition and superconductivity in LiP15

van der Waals (vdW) materials have always been a fertile ground for exploring pressure-tuned dimensional crossover and novel physical properties, such as superconductivity. Nevertheless, few of the dimensional crossovers with an accompanying superconductivity occur at a low pressure close to environmental conditions. Here, we predicted a novel $C2/m$ structure of ${\mathrm{LiP}}₁₅$ which can be achieved by compressing the experimentally synthesized $P\overline{1}$ structure above 2 GPa. Under compression, ${\mathrm{LiP}}₁₅$ goes through a one- to quasi-two-dimensional crossover and an accompanying semiconducting to superconducting electronic transition. The $C2/m$ structure is a superconductor with an increasing transition temperature under pressure that reaches up to 14.2 K at 15 GPa. In addition, we found that the phosphorus allotrope obtained by removing lithium atoms in the $C2/m$ structure is dynamically stable, and its energy is almost equivalent to that of black phosphorus at ambient pressure. These findings not only pave a new way to design a new vdW material by means of pressure-induced dimensional crossover, but also provide a possibility for searching a new vdW superconductor in similar systems.