Smartphones have many characteristics that make them attractive to adolescents. Recent work, however, has established a link between smartphone-related problems and self-esteem (self-worth) and social anxiety (fear of social relations). To date, little is known about the characteristics underpinning these relationships in combination. Driven by theory, the present study examined the association between self-esteem, social anxiety, and nomophobia (fear of being without access to a smartphone) and problem smartphone use (PSU) in a sample of early adolescents (N = 254). Self-esteem (Rosenberg Self Esteem Scale), social anxiety (Social Avoidance and Distress Scale) and their combined contribution (self-esteem x social anxiety) served as predictor variables of nomophobia (Nomophobia Questionnaire) and PSU (Mobile Phone Problem Usage Scale -- Adolescent version) in separate moderated regression analyses. We found that lower self-esteem was associated with higher nomophobia and PSU, and that higher self-esteem may be a protective factor in those lower in social anxiety, such that they are not prone to excessive smartphone use. Our findings offer preliminary markers for psychologists addressing the challenges related to excessive smartphone use in this age group.