The aim of this study was to examine ultra-short-term and short-term heart rate variability (HRV) in under-20 (U-20) national futsal players during pre-tournament training camps and an official tournament. Fourteen male U-20 national futsal players (age = 18.07 +/- 0.73 yrs; height = 169.57 +/- 8.40 cm; body weight = 64.51 +/- 12.19 kg; body fat = 12.42% +/- 3.18%) were recruited to participate in this study. Early morning 10 min resting HRV, Borg CR-10 scale session rating of perceived exertion (sRPE), and general wellness questionnaire were used to evaluate autonomic function, training load, and recovery status, respectively. Log-transformed root mean square of successive normal-to-normal interval differences (LnRMSSD) was used to compare the first 30 s, first 1 min, first 2 min, first 3 min, and first 4 min with standard 5 min LnRMSSD. Mean (LnRMSSD(mean)) and coefficient of variation (LnRMSSD(cv)) of LnRMSSD were used to compare the different time segments of HRV analysis. The result of LnRMSSD(mean) showed nearly perfect reliability and relatively small bias in all comparisons. In contrast, LnRMSSD(cv) showed nearly perfect reliability and relatively small bias from 2-4 min time segments in all study periods. In conclusion, for accuracy of HRV measures, 30 s or 1 min ultra-short-term record of LnRMSSD(mean) and short-term record of LnRMSSD(cv) of at least 2 min during the training camps are recommended in U-20 national futsal players.