We examined environmental activist behaviour as a function of two different forms of environmental identity: identification with nature (i.e., seeing oneself as part of nature, more often referred to as connectedness to nature) and politicized environmental identity (i.e., seeing oneself as a part of a collective struggle to protect the environment). We extend prior work that found politicized environmental identity mediates a relationship between identification with nature and self-reports of environmental activism (Schmitt, Mackay, et al., 2019) by using observed behavioural indicators of environmental activism: taking pro-environmental buttons (Study 1), donating to an environmental organization (Study 2), or writing pro-environmental letters to politicians (Study 3). In Study 2 the relationship between identification with nature and activist behaviour was fully mediated by politicized environmental identity. In Studies 1 and 3, politicized environmental identity partially mediated a relationship between identification with nature and observed activist behaviour. We estimated the mean effect size of the mediation pathways in all three studies using meta-analysis, which supported the mediation model.