Background: There is currently a shift in mental health research from preventing and treating mental illness to fostering positive states of mental health (PMH). Physical activity (PA) and cardiorespiratory fitness (CRF) represent modifiable protective factors that are related to physical health and disease outcomes; however, less is known about their unique associations with PMH. This cross-sectional study aimed to explore the associations between PA, CRF, and PMH in a nationally representative sample of Canadian adults. Methods: Data from 4612 adults (Mean age=41.2 years, range=18-69; 53.1% female) who participated in cycle 1 or 2 of the Canadian Health Measures Survey were analyzed. The hypothesized mediation process between self-reported PA, CRF (measured using the modified Canadian Aerobic Fitness Test), and self-reported PMH (measured by life satisfaction, happiness, and self-rated mental health) was tested using structural equation modelling. Results: The fit of the hypothesized mediational model was acceptable [X^2(4)=6.258, p=.181, CFI=.999, RMSEA [90%CI]=.011 [.000,.027], SRMR=.008). Based on the estimated standardized parameters, PA was directly associated with CRF (beta=.165, p<.001), and directly (beta=.147, p<.001) and indirectly (beta=.008, p<.05) associated with PMH. CRF was directly associated with PMH (beta=.046, p<.01). Conclusions: Interventions targeting PA and CRF may help to foster PMH and should be evaluated in future research. Considering the low magnitude of the indirect association between PA and PMH via CRF, additional physical and psychosocial factors (e.g., body mass/composition, self-esteem, mastery, confidence, social support) should be tested as mediators to better understand the processes through which PA can promote PMH.