Youth athletes demonstrate both prosocial and antisocial behaviours during sport, which may influence their own and others' enjoyment and continued involvement in sport. According to social cognitive theory, athletes' behaviour can be influenced by their social setting, values, and experiences. Youth athletes may thus be influenced to behave prosocially or antisocially by how their parents behave on the sidelines. Qualitative research has indicated that parents' behaviours do have an impact on youth athlete behaviour, but the strength of that relationship is unknown. Therefore, the purpose of the present study was to quantitatively examine the relationship between parent positive and negative behaviours and youth athlete prosocial and antisocial behaviours in sport settings. Australian youth athletes (n = 71) completed a cross-sectional online survey where they reported their parents, as well as their own, behaviours at sporting events during the past month. Results indicated that parent's positive behaviours predicted youth prosocial-teammate behaviour (? = .48, p = .000) and prosocial-opponent behaviour (? = .41, p = .001), whereas parent's negative behaviour engagement predicted youth antisocial-teammate behaviour (? = .32, p = .007) and antisocial-opponent behaviour (? = .42, p = .000). In other words, the more youth perceived their parents engaged in positive behaviours, the more they also reported engaging in prosocial behaviours; with a similar relationship found for negative parent behaviour and youth athlete antisocial behaviour. Results suggest that to reduce antisocial athlete behaviours, sport organisations may wish to target, and ultimately eliminate, negative parent behaviours.