<p>Cost-benefit analyses of whether environmental scientists should engage in activism currently rest on a thin empirical base, despite a lively debate on the topic. There are several potential benefits of scientists’ activism, but some have argued that these benefits might be offset by the potential for activism to undermine public perception of environmental scientists as unbiased and competent. To explore these potential consequences, we asked participants to read two (ostensibly real) profiles of climate scientists that either described themselves as activists or not. Study 1 (<i>N</i> = 491) found that a scientist who engaged in conventional activism was seen as slightly less competent and more hypocritical than a scientist who engaged in public science communication, but there was no impact on their persuasiveness. Study 2 (<i>N</i> = 636, pre-registered) found that a scientist who engaged in civil disobedience, a more disruptive form of activism, was seen as less competent and more hypocritical than a non-activist scientist who only engaged in teaching and research, with predicted spill-over effects on trust in the scientist’s field. Scientist activists were also downgraded on a range of other dimensions. We conclude that engaging in activism has small but reliable costs for environmental scientists.</p>