Poor mental health among young Danish women (16-34 years) constitutes a public health problem. Literature shows a positive association between physical activity and mental health, but knowledge about the association among young Danish women is missing.Aim: To investigate whether physical activity is associated with higher odds for mental health among Danish women aged 16-34 years and to investigate the significance of the duration of physical activity. Methods:This cross-sectional study included 756 16-34-year-old women from the national survey Sports participation among the Danes 2020. Physical activity was measured as the total number of minutes/week spent on sports/exercise. Women were categorised as inadequately and adequately physically active with a cut-off at 210 minutes/week. Mental health was measured through the WHO-5 Well-Being Index. Logistic regressions were performed followed by Wald tests. Further, a linear regression was performed followed by a t-test.Results: Inadequately physically active women had 38% lower odds for high mental health (OR 0.62 (CI 0.43; 0.87)). This was statistically significant. The significance of the duration of physical activity showed a statistically significant association with a coefficient of 0.013 (CI 0.007; 0.019). Conclusion:There was a statistically significant association between physical activity and mental health among Danish women aged 16-34 years. Further, there was a statistically significant dose-response association between the duration of physical activity and mental health.