The ankle range of motion (ROM) limitation increases fall risk. High reliability methods for the ankle ROM measurements require specific instrumentation or the weightbearing position—often problematic in older patients. This study tested the reliability of a new approach to measuring the ankle ROM in the supine position. In total, 256 measurements were performed on older inpatients with orthopedic problems. Intra- and interrater reliability were high for dorsiflexion (intraclass correlation coefficient [ICC]: 0.899-0.967) and moderate to high for plantar flexion measurements (ICC: 0.662-0.956). Our approach allows for precise measurement of the ankle ROM and could be useful in evaluation of the fall risk and rehabilitation progress in older patients.