The present research integrates core aspects of social identity theory with the theory of planned behaviour to investigate factors influencing engagement in sustainable agricultural practices. Using a two-wave prospective design, two studies were conducted with samples of farmers (N = 609 and N = 259, respectively). At Time 1, a questionnaire survey assessed theory of planned behaviour variables in relation to engaging in riparian zone management (a sustainable agricultural practice). In addition, intergroup perceptions (i.e. relations between rural and urban groups), group norms and group identification were assessed. At Time 2, self-reported behaviour was measured. There was support for the integrated model across both studies. As predicted, past behaviour, attitudes and perceived behavioural control were significant predictors of intentions, and intentions significantly predicted self-reported behaviour. Group norms and intergroup perceptions were also significant predictors of intentions providing support for the inclusion of social identity concepts in the theory of planned behaviour. More supportive group norms were associated with higher intentions, especially for high-group identifiers. In contrast, more negative intergroup perceptions were associated with lower intentions and, unexpectedly, this effect only emerged for low-group identifiers. This suggests that in the context of decisions to engage in riparian zone management, an important sustainable agricultural practice, high identifiers are influenced predominantly by in-group rather than out-group considerations, whereas low identifiers may attend to cues from both the in-group and the out-group when making their decisions.