Background: The relation between work motivation and well-being are relatively well theoretically entrenched, but few studies have been conducted on nurses within psychiatry. Objective: The aim of the present study was to examine the differences in intrinsic work motivation (autonomy, competence, relatedness), mental health and burnout between young (≤40 years) and older (>40 years) nurses employed in psychiatry. Method: The study was an observational study with a cross-sectional design and group comparisons, conducted as a survey. The sample constituted of 60 employed nurses. Chi2 analysis, ANOVA, t-test and linear regression analyses were conducted in SPSS (22,0). Result: Nurses had a relatively high intrinsic work motivation. Autonomy (M = 4,73) was estimated significantly lowest. Moreover, autonomy was estimated significantly higher in the younger (M = 5,02, SD = 0,77) than in the older (M = 4,55, SD = 0,81) group. There was a significant relationship between competence and mental well-being in both groups and a significant relationship between competence and burnout in the older group. Conclusion: The study showed that there were differences in how older and younger nurses reported motivation. Thus, in order to improve mental health among nurses it is necessary to focus on competence development in the work.