Cooperative management of shared natural resources is one of the most urgent challenges the world is facing today. Addressing this challenge requires understanding of group process involved in willingness to cooperate across cultural contexts. We explore this issue in the context of shared land management among agro-pastoralists in Northern Tanzania. In an experimental study (N = 209) we tested whether salience of community (rather than household) identity and shared threat from land degradation is related to stronger willingness to protect communal land. The study used a mixed design with salience of community vs household identity as a between-subject IV (with random assignment), and outcome measures completed before and after the experimental manipulation (within-subject IV). Dependent variables included community efficacy and perceived support, intentions to protect communal (and household) land, and perceived community norms related to communal land protection. Participants were also asked to divide a fixed number of seeds between their household and village communal land. The results demonstrated that there was a significant interaction between the experimental condition and time (pre vs post) on community identification, perceived community support and efficacy, trust, and intentions to protect communal land - post manipulation (but not pre-manipulation) participants in the community condition reported higher scores on these outcomes than those in the household condition. In addition, participants in the community condition donated more seeds for planting on the communal land compared to those in the household condition. The results offer a critical field-based validation of the social identity approach to supporting cooperation around shared resources and suggest practical implications for promoting sustainable resource management.