The aim of this research is to determine which characteristics of group cohesion are mostly related to the psychosocial well-being of handball players and to identify specific behaviours of coaches that are related to a higher degree of group cohesion of a sports team. The research included 251 participants, players of official Croatian handball leagues from 19 Croatian handball clubs. The age range of participants was between 18 and 42 years, and participants were on average 22 years old (M =22.44; SD =4,27). Data were collected using the paper-pencil method, and the instruments used in this research are Leadership Scale for Sport Questionnaire (Chelladurai and Saleh, 1980; Chelladurai, 1989), Coach's Negative Behaviour Questionnaire (Greblo Jurakić and Keresteš, 2017), The Mental Health Questionnaire (Vuletić et al., 2018) and the Youth Sports Environment Questionnaire (Eys et al., 2009). The results of the research showed that athletes who compete on the higher and lower levels of sport competitions differ only on the social support subscale. Namely, athletes who compete on higher levels of competition perceive less social support compared to those who compete on lower levels. As expected, both dimensions of group cohesion were statistically significantly positively associated with positive coach's behaviours and the coach’s focus on results, and statistically significantly negatively associated with the coach’s perception of insensitivity to the personal well-being of athletes. In addition, a low statistically significant negative correlation was found between task-oriented group cohesion and negative feedback from coaches. Also, a significant positive correlation of the dimensions of group cohesion with the social and psychological well-being of athletes was found. The perception of the coach's behaviour explains 11% of the variance of task-oriented group cohesion and 20% of the variance of social group cohesion. Finally, hierarchical regression analysis found that group cohesion explains 23% of the variance of social well-being and 17% of the variance of psychological wellbeing of athletes, in addition to the coach's behaviour. The results of the research contribute to a better understanding of the relationship between coach's behaviour and group cohesion of a sports team and how group cohesion is related to the psychosocial well-being of athletes.