Burnout is a psychosocial construct that has been extensively studied in athletes. Burnout is a significant threat to an athlete’s mental health and increases the risk of dropout from sport (Gustafsson, Madigan, & Lundkvist, 2017). However, little is known about prevalence of clinical-levels of burnout in athletes. This study therefore aimed to estimate the prevalence of athlete burnout based on clinical cut-off scores taken from the Shirom Melamed Burnout Questionnaire (SMBQ). To do so, cut-off scores were converted from the SMBQ to the Athlete Burnout Questionnaire (ABQ). This provided a cut-off score of 3.14. Data were derived from one cross-sectional sample (N = 1,931) and one 10-week longitudinal sample (N = 186; 10 waves). Analyses focused on the physical and emotional exhaustion subscale of the ABQ. In the longitudinal sample, the stability of the cut-offs was examined (i.e., those athletes scoring over 3.14 for over 50% of the time-points, versus those who never scored above 3.14). For the cross-sectional sample, 13% of athletes scored above the clinical cut-off score. For the longitudinal sample, 7.1% of athletes scored above the cut-off on over 50% of occasions, while 77.6% of athletes never scored above the cut-off score on any occasion. The present findings suggest that the estimated prevalence of athlete burnout differs when cross-sectional data are analysed than when longitudinal data are utilised. The findings reiterate that a significant proportion of athletes are at risk of burnout and that prevalence seem to be similar comparing with student- and working populations.