The study explored the role of social media in shaping millennial leadership styles for ethical decision-making in tertiary institutions in Ogun State. Two research questions and two hypotheses guided the study. A sample size of approximately 311 millennial leaders were selected from various tertiary institutions (universities, polytechnics, and colleges of education) in Ogun State. A structured questionnaire validated by three experts with a reliability value of 0.81 was used for data collection. The data collected were used to answer the research questions and the hypotheses. Specifically, the research questions were analysed using coefficient of determination, while the hypotheses were tested using regression analysis at 0.05 threshold level. The findings revealed a: significant relationship between social media usage and leadership styles of millennials in tertiary institutions in Ogun State; and significant relationship between social media usage and ethical decision-making processes of millennial leaders in tertiary institutions in Ogun State. The study concludes that increased engagement with social media platforms correlates with more effective and ethically-oriented leadership styles among millennial leaders. It was recommended that tertiary institutions should incorporate social media literacy and ethical usage training into their leadership development programs, and tertiary institutions should create and promote online platforms or forums where millennial leaders can engage in discussions about ethical dilemmas and leadership challenges.