<p>Diamond is known as the ultimate semiconductor owing to its excellent physical properties. However, the high difficulty of n-type doping and the poor electrical performance of n-type diamonds remain major challenges for the application of diamond semiconductor materials. In this paper, a high-pressure thermal diffusion method for the n-type doping of diamond, which utilizes high pressure to reduce the volume difference between phosphorus atoms and carbon atoms, is reported for the first time. This method can achieve efficient doping and ionization of phosphorus atoms at the lattice sites of diamond. The prepared phosphorus-doped diamond exhibited the lowest resistivity (2 Ω cm) and highest electron concentration (2.27 × 10¹⁸ cm⁻³) observed in any known phosphorus-doped diamond single crystal at room temperature (300 K). The high-pressure thermal diffusion method provides an effective approach for diamond n-type doping, which may play an important role in the design and preparation of future diamond-based semiconductor devices.</p>