Brand attachment shows significant effects on the adoption of behaviours in the profit domain. Surprisingly, however, this concept has not been tested in the non-profit context. Concepts such as brand image or brand personality have already been the subject of studies, but attachment to nonprofit brand remains a concept to be explored. This study proposes to fill this gap by testing the determinants and effects of attachment to the nonprofit brand on three pro-environmental behaviours. To this end, a study of 1160 supporters of an environmental NGO was carried out. This study leads to the identification of the formation of the concept of attachment to the nonprofit brand and the measurement of its effects. It also questions the effects of the concepts of nonprofit brand image and value orientations.