Social network diversity can be associated with physical and mental health among adolescents, which might be modified by their perceived degree of influence in class. We aimed to examine the association between social network diversity and physical and mental health, and to elucidate its effect modification by perceived degree of influence in class. Data were obtained from the Kochi Child Health Impact of Living Difficulty (K-CHILD) study in 2016, which targeted 5th, 8th and 11th grade children living in Kochi Prefecture in Japan (N = 9998). Social network diversity accounted for the number of social roles in which adolescents had regular contact. Degree of influence in class, depression (using Depression Self-Rating Scale for children (DSRS)) and self-rated health were assessed by children, and behavior problem and prosocial behavior (using Strengths and Difficulties Questionnaire (SDQ)) was assessed by caregivers. Significant association of social network diversity with depression (Coefficient (B) = -0.59, 95% confidence interval (CI) = -0.68 to -0.50), self-rated health (B = 0.09, 95% CI = 0.07 to 0.11), behavior problem (B = -0.71, 95% CI = -0.82 to -0.61) and prosocial behavior (B = 0.06, 95% CI = 0.02 to 0.11) were found. The association with depression and self-rated health was stronger among children with perceived low degree of influence (both p for interaction < 0.001). A similar trend was observed for behavior problem (p for interaction = 0.053), but effect modification was not found for the association between social network diversity and prosocial behavior. Social network diversity was beneficial for adolescent physical and mental health, especially for children with perceived lower degree of influence in class. Copyright © 2020 The Author(s). Published by Elsevier Ltd.. All rights reserved.