Background: Individual differences in personality, behavioural, and academic outcomes of gifted adolescents remain under-explored. Aims: The present study directly compared selected and unselected adolescents on multiple measures of personality, behavioural strengths and difficulties, and achievement. Sample: Nine hundred seventy-three adolescents selected for high performance in Science, Technology, Engineering, and Mathematical (STEM) fields (M = 15.23; SD = 1.11), and one thousand two hundred sixty-one unselected adolescents (M = 15.07; SD = 1.18) participated in the study. Methods: Participants completed self-report measures that assess the Big Five, the Dark Triad, and Behavioural Strengths and Difficulties. Demographic information and academic achievement in Maths and Russian were also obtained. Results: The observed differences in personality and behaviour traits between selected and unselected samples were negligible as measured by ANOVAs. The selected sample had on average slightly lower scores on conscientiousness, extraversion, agreeableness, openness to experience, and subclinical narcissism, partial Eta Squared (ES) = [0.01 to 0.05]; slightly lower scores on prosocial behaviour; and slightly higher scores on internalizing and externalizing problems, ES = [0.01 to 0.04]. The selected group also showed higher Year and Examination grades (ES = 0.05 and 0.23, respectively). However, MANOVA results showed larger differences between samples (ES = 0.15). Conclusion: Our results showed no pronounced differences between selected and unselected samples in any trait apart from examination performance. However, multivariate results suggest greater overall differences. These results suggest that high-achieving individuals may be characterized by specific combinations of personality and behavioural traits.