Objective: The molecular genetic of personality disorders has been investigated in several studies; however, the association of antisocial behaviours with synaptosomal-associated protein 25 (SNAP25) gene polymorphisms has not. This association is of interest as SNAP25 gene polymorphism has been associated with attention-deficit hyperactivity disorder and personality. Methods: We compared the distribution of DdeI and MnlI polymorphisms in 91 young male offenders and in 38 sex-matched healthy control subjects. We also investigated the association of SNAP25 gene polymorphisms with severity of psychopathy and with temperament traits: novelty seeking, harm avoidance, and reward dependence. Results: The MnlI T/T and DdeI T/T genotypes were more frequently present in male subjects with antisocial personality disorder (APD) than in sex-matched healthy control subjects. The association was stronger when the frequency of both DdeI and MnlI T/T were taken into account. In the APD group, the genotype was not significantly associated with the Psychopathy Checklist–Revised scores, measuring the severity of psychopathy. However, the APD subjects with the MnlI T/T genotype had higher novelty seeking scores; whereas, subjects with the DdeI T/T genotype had lower reward dependence scores. Again, the association between genotype and novelty seeking was stronger when both DdeI and MnlI genotypes were taken into account. Conclusion: DdeI and MnlI T/T genotypes may be a risk factor for antisocial behaviours. The association of the SNAP25 DdeI T/T and MnlI T/T genotypes with lower reward dependence and higher novelty seeking suggested that SNAP25 genotype might influence other personality disorders, as well.