Background: Healthcare professionals experience high levels of stress due to demanding work, especially in metropolitan areas. Nature-based interventions offer potential mental health benefits. This randomized intervention study aimed to evaluate the effects of nature immersion therapies on mental health outcomes in healthcare workers with different psychological risk in Bogota, Colombia. Methods: During a period of 6 months, a total of 82 healthcare workers from two institutions were assigned to three groups: two exposed weekly to nature (parks and forests) and one control group with monthly conventional interventions. Psychological assessments of stress, anxiety, fatigue, and sleep quality were conducted at three time points (baseline, three, and six months of intervention). Cortisol Awakening Response (CAR) was measured monthly using immunoassay. Results: A decrease in the proportion of participants reporting high levels of perceived stress was observed in both intervention groups. Both forest and parks interventions significantly reduced anxiety and fatigue, while sleep quality improved only in the forest group. Multivariate analysis found a negative association between fatigue and forest intervention, as well as significant differences in CAR concentrations across groups over time. Conclusions: This study provides evidence that nature immersion therapy, particularly urban forests, positively impact mental and physical health, reducing stress, anxiety, fatigue, and CAR levels, and could be considered as an effective intervention to enhance workers’ resilience to stress, benefiting their overall health and well-being.