In this study an algorithm designed for the diagonal stride in classical cross-country skiing was adapted to compute spatio-temporal parameters for uphill ski mountaineering using a ski fixed inertial sensor. Cycle duration, thrust duration, cycle speed, cycle distance, elevation gain, and slope angle were computed and validated against a marker-based motion capture system during indoor treadmill skiing. Skiing movement of 12 experienced, recreational level athletes was measured for nine different speed and slope angle combinations. The accuracy (i.e. mean error) and precision (i.e. standard deviation of the error) were below 3 ms and 13 ms for the cycle duration and thrust duration, respectively. Accuracy (precision) for cycle speed, cycle distance and elevation gain were −0.013 m/s (0.032 m/s), −0.027 m (0.018 m), and 0.006 m (0.011 m), respectively. Slope angle accuracy and precision were 0.40° and 0.32°, respectively. If the cross-country skiing algorithm would be used without adaptations, errors would be up to one order of magnitude larger. The adapted algorithm proved valid for measuring spatio-temporal parameters for ski-mountaineering on treadmill. It is expected that the algorithm shows similar performance on snow.