The psychology of sustainability and sustainable development aims to study the personal characteristics that promote effective and sustainable well-being for individuals and environments from a psychological research perspective. According to the self-determination theory, the psychological need for relatedness is positively associated with happiness and flourishing. In turn, emotional intelligence, i.e., understanding and managing one’s own emotions and recognizing others’ emotions, may play a key role in this association. Therefore, the present study investigates the mediating role of emotional intelligence in the relationship between need for relatedness and both happiness and flourishing. Basic Psychological Needs scales (BPNs), Emotional Intelligence Scale (EIS), Flourishing Scale, and Happiness Scale were administered to 216 Italian participants (age range 15–66 years old). A mediation model via a structural equation model for path analysis was tested. The results showed that the psychological need for relatedness positively associated with both happiness and flourishing and that emotional intelligence mediated these associations. These results suggest that important interventions may be performed to promote flourishing and happiness, enhancing emotional intelligence through specific training differently from need for relatedness that, instead, can be considered substantially stable.