Objective: To prevent and intervene in the issue of smartphone addiction among adults, this study aims to explore the mediating role of loneliness in the relationship between fear of missing out (FoMO) and smartphone addiction. Method: A survey was conducted involving 1,371 adults in Sichuan Province using the FoMO Scale, the Loneliness Scale, and the Smartphone Addiction Scale. Results: The findings indicate that: (1) There are significant positive correlations among adults’ FoMO, loneliness, and smartphone addiction; (2) FoMO can directly predict smartphone addiction positively in adults, and it can also indirectly predict smartphone addiction by influencing loneliness. Conclusion: Loneliness plays a partial mediating role in the relationship between adults' FoMO and their smartphone addiction.