Social media mindsets are the core beliefs that orient individuals’ expectations, behaviors, attributions, and goals about social media’s role in their lives. In four survey studies (N = 2,179), we show people hold distinct mindsets about the amount of agency they have over their social media use (“in control” vs. “out of control”) and the valence of its effects (“enhancing” vs. “harmful”) that are meaningfully related to psychological well-being. We develop and apply the Social Media Mindsets scale, revealing that agentic, positive mindsets are associated with better well-being and low-agency, and negative mindsets are associated with worse well-being (Studies 1, 2a, and 2b). Notably, these mindsets explained more variance in relational well-being and psychological distress than other measures (Study 3) and were related to differences in how people used social media and interpreted the time they spent on it (Studies 3 and 4). Our findings introduce a novel potential explanation for heterogeneous social media effects on well-being.