Nowadays, marketing is entering a new era of brand activism. How to select appropriate activism issues is the key to the success of brand activism strategy. However, little is known about the relationship between brand and activism issue fit and consumer brand attitudes. Drawing on the two-process model of similarity, this paper explores the impact of brand and activism issue fit (taxonomic vs. thematic) on consumer brand attitudes and its psychological mechanism. Specifically, based on secondary data and two laboratory experiments, the study finds that brand activism based on thematic (vs. taxonomic) fit is more effective in improving consumer brand attitudes and the mechanism of this effect is the perceived purity of justice motives of brand activism. This discrepancy in perceived justice motives is affected by the perception of benefit and loss. When consumers perceive losses, they will question the purity of justice motives in brand activism, even if the brand demonstrates higher pro-social efforts; whereas when consumers perceive benefits, it strengthens their perception of the justice motive purity of brand activism. The conclusions not only enrich the research on brand and activism issue fit and justice motives, but also provide a series of significant management implications for marketers to leverage brand activism strategies to obtain consumers’ positive brand attitudes.