Background: Previous research into effects of personality on internet addiction in adolescents has produced inconclusive results. Adolescents are considered to be at risk of Internet addiction because of their physical and psychological immaturity and negative effects of Internet use are more obvious in this age group. Aim: To determine the characteristics of adolescent personality that are most often associated with normal, moderate or serious Internet addiction and how these characteristics reflect on the six dimensions of Internet addiction. Methods: The study included 1078 male and female adolescents aged 11-18 years from Croatia, Poland, and Finland. The Internet addiction was assessed using Young's Internet Addition Test. A 30-item five-factor personality inventory, i.e., a Croatian version of the Goldberg's International Personality Item Pool, was used for personality assessment. Data were analyzed using a correlation analysis. Results: The level of Internet addiction did not significantly vary across different levels of neuroticism (P<.001); extraversion (P=.053); openness to experience (P<.001), and conscientiousness (P=.022). High and very high levels of openness were found in (46/241, 19%) of adolescents without Internet addiction and (92/241, 38%) adolescents with moderate and serious Internet addiction. Thus, openness was more pronounced in those more addicted to the Internet. Adolescents with moderate and serious Internet addiction showed higher levels of neuroticism, openness to new experience, and conscientiousness in comparison with adolescents evaluated as normal Internet users, whereas no difference was found for extraversion and agreeableness (P>.05). Conclusions: Analysis of correlation between five big personality traits and Internet addiction indicated that neuroticism, extraversion, and openness to new experience were more common among adolescents with moderate and serious Internet addiction.