Given the popularity of social media and the growing presence of these tools in the daily lives of individuals, research about the elements that can be linked to their problematic use appears to be of great importance. The objective of this study was to investigate the factors that may contribute to the levels of social media addiction, by focusing on the role of alexithymia, body image concern, and self-esteem, controlled for age and gender. A sample of 437 social media users (32.5% men, 67.5% women; Mage = 33.44 years, SD = 13.284) completed an online survey, including the Bergen Social Media Addiction Scale, Body Image Concern Inventory, Rosenberg Self-Esteem Scale, and Twenty-Item Toronto Alexithymia Scale, together with a demographic questionnaire. Results showed a significant association between alexithymia and social media addiction, with the total mediation of body image concern (and more in detail, body dissatisfaction) and the significant moderation of self-esteem. Gender and age showed significant effects in these relationships. Such findings may offer further insights into the field of clinical research on social media addiction and may provide useful information for effective clinical practice.