Limited ankle dorsiflexion is closely related to important foot and ankle pathologies. Various measurement devices and methods have been examined, but these have demonstrated limited validity and reliability. The purpose of the present study was to assess the validity and intra- and interobserver reliability of the Achillometer®. A total of 22 consecutive subjects with ankle or foot pathologies and 39 healthy participants were included. All participants were measured using the goniometer and the Achillometer®, a portable device used to assess ankle dorsiflexion in the weightbearing position with knee in extension. The intraclass correlation coefficient, standard error of the mean, and minimal detectable change were determined. The goniometer and Achillometer® revealed high intraobserver reliability coefficients with an intraclass correlation coefficient of 0.88 (standard error of the mean 1.49, minimal detectable change 4.12) and 0.85 (standard error of the mean 1.57, minimal detectable change 4.34), respectively. The interobserver reliability of both measurement techniques ranged from 0.63 to 0.67. The Achillometer® showed a strong correlation with the goniometer for both observers. In conclusion, the Achillometer® is a valid measurement device to assess ankle dorsiflexion range of motion in the weightbearing position with an extended knee in a heterogeneous population. The device has good intraobserver and moderate interobserver reliability and measurement properties comparable to those of the goniometer.