Purpose. The purpose of the study was to address how assertiveness relates to bullying behaviour among adult prisoners.Methods. The sample was selected from six separate prison establishments (three male, three female) and consisted of 502 adult prisoners (285 men, 217 women). Prisoners were placed into one of four bully‐categories (‘pure bullies’, ‘bully/victims’, ‘pure victims’ or ‘not involved’) on the basis of a self‐report behavioural checklist (DIPC: Direct and Indirect Prisoner Behaviour Checklist). They also completed the Rathus Assertiveness Schedule (RAS).Results. Assertiveness among prisoners was made up of three components: social assertiveness, argumentative and combativeness, and a willingness to converse with others. Men reported significantly higher overall assertiveness and social assertiveness scores than women. Pure victims reported lower total assertiveness scores than the other categories and there was a trend for pure bullies to report higher total assertiveness scores than the other bully‐categories. Bully/victims scored significantly higher on the argumentative and combative scale in comparison to the overall mean and there was a trend in the same direction for pure bullies. There was a trend for pure victims to score lower on this scale and those not involved in bullying reported significantly lower scores on this scale. Those not involved also scored significantly higher on social assertiveness and there was a trend for both bully/victims and pure victims to score lower on this component.Conclusions. The present research showed how certain components of assertiveness relate to bullying behaviour among prisoners. The findings are discussed in relation to previous research, and possible implications for intervention are outlined.