Cultivating an eudaimonic lifestyle benefits mental health. However, it is not known under which circumstances the benefits of eudaimonia are the most pronounced. This cross-sectional study aimed to examine psychological needs (autonomy, competence, and relatedness) as the moderators of the effects eudaimonia has on mental health (general mental health, positive and negative mental health). In total, 328 young adults took part in this study, of which 36.6% were males, whose ages ranged between 18 and 30, with a median of 19 years (SD = 1.32). Questionnaires were used to measure eudaimonia (The Eudaimonia Scale from The Hedonic and Eudaimonic Motives for Activities), psychological needs (The Basic Psychological Needs Scale), and mental health (The Mental Health Inventory). To analyze the data, hierarchical regression analyses and analyses of variance have been performed. The results revealed that of all psychological needs, only the need for relatedness moderated the relationship between eudaimonia and general mental health and eudaimonia and negative mental health. Individuals with high eudaimonia and a highly satisfied need for relatedness had good general mental health and fewer mental health disturbances. The findings suggest that if one practices meaningful behaviors, the positive change in the mental health domain is greatest when those actions benefit not only this individual but others as well.