Wearable devices enable RR interval (RRi) measurements during various conditions. We aimed to assess the validity of the Polar H10 for RRi acquisition during pre-rest stabilization, rest, sympathetic nervous system activity stimulation and recovery conditions for heart rate variability (HRV) analysis in ski mountaineers. RRi were simultaneously obtained via electrocardiogram and the Polar H10 with V800 wrist-watch among eleven elite athletes in the supine position. Short-term (5-min) and ultra-short-term (1-min) heart rate (HR) and HRV parameters were analyzed. Mean absolute percentage difference between parameters from different devices ranged from 0% to 5.4%. Intraclass correlation and concordance correlations ranged between 0.76 and 1.00. Limits of agreement (LoA) for short-term measures did not exceed the defined maximum acceptable difference (SWC) through all conditions for mean RRi, HR, time-domain and nonlinear indices, and for frequency-domain (fast-Fourier-related) during the rest condition. LoA exceeded the SWC for mean, minimal and maximal HR, time-domain and nonlinear parameters from select 1-minute samples in all conditions. The Polar H10 provides RRi that could be used for short-and ultra-short-term HRV analysis from stable resting conditions in elite ski mountaineers. Ultra-short-term parameters from exercise should be analyzed with caution especially from later minutes of activity.