Patient-specific instrumentation (PSI) can be applied in unicompartmental knee arthroplasty (UKA) to assist surgeons in planning and positioning prostheses. This study aims to compare the accuracy of prosthesis positioning in medial UKA performed with a newly designed PSI tool to those of UKA performed with robotic assistance and previously reported PSI-assisted techniques.Ninety-one patients underwent medial mobile-bearing UKA with the new PSI tool designed based on preoperative CT scans from May 2023 to June 2024. Deviations between planned and actual postoperative positions of femoral and tibial components in the coronal, sagittal, and axial planes were measured with CT overlay, and root mean square error (RMSE) was calculated. Accuracy was benchmarked by comparing the proportion of deviations within 2° with published robotic-assisted UKA data. Median absolute deviation (MAD) was compared with those of previous PSI studies. Cases of prosthesis size adjustment and PSI failure were also recorded.RMSE values for femoral component angle deviations were 2.0°, 3.7°, and 2.8° in coronal, sagittal, and axial planes; for the tibial component, values were 1.5°, 1.8°, and 2.6°, respectively. MAD values were consistently low, and angular deviations were generally comparable or superior to robotic-assisted UKA. In 4 cases (4.4%), prosthetic size was adjusted intraoperatively.The newly designed PSI tool effectively achieves the preoperative planning goals in medial mobile-bearing UKA. When compared with the published results of robotic-assisted and PSI-guided UKA, this tool provides comparable or even superior component positioning accuracy, demonstrating its potentials for wider clinical applications.Copyright © 2025 The Author(s). Published by Wolters Kluwer Health, Inc.