This study assessed the Stryd running power meter validity at sub-maximal speeds (8 to 19 km/h). Six recreational runners performed an incremental indoor running test. Power output (PO), ground contact time (GCT) and leg spring stiffness (LSS) were compared to reference measures recorded by portable metabolic analyser, force platforms and motion capture system. A Bayesian framework was conducted for systems validity and comparisons. We observed strong and positive linear relationships between Stryd PO and oxygen consumption (R-2 = 0.82, BF10 > 100), and between Stryd PO and external mechanical power (R-2 = 0.88, BF10 > 100). Stryd power meter underestimated PO (BF10 > 100) whereas GCT and LSS values did not show any significant differences with the reference measures (BF10 = 0.008, BF10 = 0.007, respectively). We conclude that the Stryd power meter provides valid measures of GCT and LSS but underestimates the absolute values of PO.