Radicalization is a process, by which individuals adopt extreme political, social and religious ideation that leads to mass violence acts. It has been hypothesized that mental health characteristics might be associated with a risk of radicalization. However, a qualitative synthesis of studies investigating the relationship between mental health and radicalization has not been performed so far. Therefore, we aimed to perform a systematic review of studies examining the association between mental health characteristics and the risk of radicalization. Two reviewers performed an independent search of online databases from their inception until 8^th April 2018 and 12 publications met eligibility criteria. There were several methodological limitations across the majority of eligible publications, including doubtful sample representativeness, use of diagnostic procedures without personal assessment of mental health status or lack of standardized tools for assessment of mental health. Representative cross-sectional studies revealed that depressive symptoms might be associated with radicalization proneness. However, it remains unknown whether depressive symptoms are associated with resilience or vulnerability to radicalization. Another finding from our systematic review is that several personality traits might predispose to develop extreme ideation. Finally, there is some evidence that lone-actors might represent a specific subgroup of subjects with extreme beliefs which can be characterized by high prevalence of psychotic and/or mood disorders. In conclusion, this systematic review indicates that caution should be taken on how the association between ‘mental health’ and ‘radicalization’ is being claimed, because of limited evidence so far, and a number of methodological limitations of studies addressing this issue.