Transition to high school is a very challenging period because adolescents are changing their social environment. It can be particularly challenging for rejection sensitive adolescents. Rejection sensitivity is a personality disposition that refers to the perception of social cues, emotions, cognitions, and behavioural reactions following implicit or direct social rejection. It could further explain how adolescents adjust to high school transition and how negative social experiences predict maladjustments. Highly rejection sensitive adolescents tend to have impaired interpersonal relationships and more emotional issues. This research aims to examine determinants and outcomes of rejection sensitivity in the period of transition to high school, i.e., correlations between parental and peer rejection, rejection sensitivity, and depressive symptoms, social anxiety symptoms and aggression. A two-wave longitudinal research was conducted with the first wave performed at the beginning of the first grade of high school, and the second wave at the beginning of the second semester of the same school year. Adolescents` self-reports, peer-reports, and parent-reports were collected. Analyses were conducted on the data collected from 886 adolescents participating at both measurement points, as well as the data collected from their mothers (N = 604) and fathers (N = 536) were analysed. Structural equation modelling was used for examining the postulated hypotheses. The results showed that rejection sensitivity was moderately stable during the transition period. Rejection sensitivity dimensions, except self- and peer-reported angry rejection sensitivity, were more stable for females. Mean-levels of self-reported, but not peer-reported, rejection sensitivity decreased between the beginning of the first grade and the beginning of the second semester. Female adolescents, in comparison to their male peers, had higher levels of self-reported anxious dimension of rejections sensitivity and the dimension of expectations of rejection, while males had higher peer-reported angry rejection sensitivity than females. Parental and peer rejection longitudinally predicted higher self- and peer-reported dimensions of rejection sensitivity, with peer rejection being a stronger predictor. Rejection sensitivity measured at the beginning of the school year predicted changes in depressive symptoms, social anxiety symptoms, and aggression over a four-month period. Peer-reported rejection sensitivity predicted all maladjustment outcomes except for the depressive symptoms. Rejection sensitivity (adolescent and peer-reported) was a mediator between peer rejection and maladjustments, while paternal (adolescent reported) and maternal (adolescent and motherreported) rejection had only direct effects on maladjustments. In all analyses, gender moderation was examined, revealing some gender-specific relations between rejection sensitivity, rejection, and maladjustments. The results of this study contribute to the existing literature on rejection sensitivity by shedding some light on its role in adolescents’ adjustment during a sensitive period of transition to high school. To the best of our knowledge, this study is the first one to examine parental rejection, peer-reported rejection sensitivity and aggression in the period of transition to high school. Besides its scientific contribution, this study may also have important practical implications, enabling the development of preventive and interventive programmes aimed at improving the psychosocial adaptation during the period of transition to high school, especially for adolescents with high level of rejection sensitivity.