Building on social movement theory, this study assesses the influence of social media activism on the stock market performance of targeted firms. We focus on information published on Twitter by two critical stakeholders: consumer associations and trade unions. To the extent that social media represent a valid medium to mobilize stakeholders' activism, protests on Twitter may damage firm reputation, leading to capital market reactions. Using a corpus of over 1.5 million tweets referring to Spanish listed banks, we study the impact of activism by looking at targeted firms' abnormal variations in price and trading volume. Our findings suggest that the Twitter activism of key stakeholders has a significant impact on investors' decisions. Further, our empirical analyses indicate that the mechanisms affecting investors' behavior differ depending on the characteristics of the stakeholder group. Hence, this study contributes to understanding how social movements influence corporate behavior via social media. Copyright © 2017 John Wiley & Sons, Ltd and ERP Environment