The social media industry is highly competitive, and positive word of mouth (PWOM) plays a key role in social media influencers (SMIs), that is, ordinary individuals who self-disclose personal experiences and opinions through social media platforms, gaining more followers. However, the relationship between self-disclosure (SD) and PWOM and the mechanisms underlying this relationship in the context of SMIs remain unclear. In this study, drawing from social penetration theory and the stimulus–organism–response paradigm, we developed a conceptual model to investigate the aforementioned mechanisms from the perspective of follower-perceived social relationships (i.e., transparency and psychological closeness). A total of 461 valid questionnaires were collected from SMI followers. Structural equation modeling was conducted to evaluate hypothesized associations. The results indicated that the SD of SMIs is associated with followers’ perceptions of the transparency and psychological closeness of these SMIs. These factors are also associated with followers’ PWOM. Mediation analysis confirmed that transparency and psychological closeness mediate the relationship between SD and PWOM; perceived transparency mediates the relationship between SD and perceived psychological closeness, and perceived psychological closeness mediates the relationship between perceived transparency and PWOM. By understanding the relationships between SD and followers’ PWOM and the mechanisms underlying these relationships from the perspectives of transparency and psychological closeness, we provide theoretical insights into the literature on SD and PWOM and offer suggestions for how SMIs can modify their SD practices to elicit PWOM from their followers.