For corporations engaged in corporate social advocacy (CSA), establishing legitimacy among publics is challenging when they take stands along clear ideological lines on controversial issues. This study examined two questions: (1) how would the congruence between individuals’ political ideologies and corporations’ CSA stances influence perceived CSA legitimacy; and (2) how would individuals’ political ideology strength moderate this relationship? A computational analysis was conducted to examine individual-level ideological congruence and perceived CSA legitimacy in six real-world CSA events. Pairing sentiment analysis with a self-trained model for individual ideology detection, this study cross-analyzed naturalistic data generated by 5181 ordinary users involved in the CSA events on Twitter. It was found that individuals’ perceptions of corporations involved in CSA were highly dependent on the extent to which their own political ideologies were congruent with the corporations’ CSA stances. Also, the influence of ideological congruence was amplified among individuals with stronger political ideologies. However, CSA legitimacy perceptions can be highly situational, with individual events potentially yielding varied results depending on factors such as the issue’s socio-historical context. This study overall demonstrates the important role that individual-level political factors can play in influencing publics’ CSA legitimacy perceptions. This study also highlights the feasibility of using cutting-edge computational methods in assessing publics’ voices on social media as a source of CSA legitimacy.