Pseudoaneurysms of the lateral plantar artery (LPA) are rare. In this case study, a very rare case of a pseudoaneurysm of the LPA, caused by a knife cut, is reported. A female adult patient presented with a growing lump in the arch of the left foot for 4 weeks. She had an accidental steak knife stab her on the arch of the left foot and had the cut sutured 5 weeks prior. The physical examination revealed a warm, pulsatile, and tender lump without a clear border deep to the scar in the lateral arch. Duplex ultrasonography (DUS) detected a cavity with the wall uneven in thickness and the outer wall undistinguished. Doppler demonstrated a typical Yin-yang sign inside the cavity and a neck-like structure connecting the cavity to an artery along the path of the LPA, suggesting that this was a LPA pseudoaneurysm, caused by penetration trauma. Later, the patient received priority surgery to have the pseudoaneurysm repaired.