This research investigates the effect of greenwashing on consumer behavior and particularly on boycott intention and negative word of mouth, while also analyzing the moderating role of environmental concern. A conceptual model was developed from the proposed hypotheses based on the theory of reasoned action. The data of this study were collected by an online survey of 322 Tunisian consumers that are familiar with fast-fashion brands. Partial least squares structural equation modeling (PLS-SEM) was used to analyze data and test the direct, mediation and moderation relationships. The results indicate that greenwashing exerts a positive impact on consumers' intention to boycott, which subsequently influences negative word-of-mouth. Moreover, boycott intentions fully mediate the linkage between greenwashing and negative word-of-mouth. Furthermore, the findings show that environmental concern significantly moderates the effect of greenwashing on negative word of mouth, along with the causal relationship between consumers’ boycott intention and negative word of mouth. This research contributes to the growing consumer behavior literature on greenwashing, as it provides new insights concerning consumer retaliation behaviors in the context of fast fashion and particularly in an emerging market in the MENA region. This study also yields a set of valuable recommendations for businesses, emphasizing the crucial role of authenticity and transparency in the corporate environmental information used by companies.