Background Potential concerns are increasing that smartphone and Internet addictions may have deleterious effects on the mental health. Despite the recognition of the important role that implicit associations may have over explicit processes in addiction, such implicit associations have not been comprehensively investigated with respect to Internet addiction. Therefore, we modified the Implicit Association Test (IAT) for smartphone and Internet addictions and investigated its validity in children and adolescents. Methods In this experimental study, 78 at-risk children and adolescents ranging in age from 7 to 17 years completed an IAT modified with pictures captured from the most popular Internet games among youth. Furthermore, measures of Internet and smartphone addictions, mental health and problem behaviors, impulsive tendencies, self-esteem, daily stress, and quality of life were assessed simultaneously. Results Significant correlations were found between IAT D2SD scores and standardized scales for Internet (r = .28, p < .05) and smartphone (r = .33, p < .01) addictions. There were no significant correlations between IAT parameters and other scales measuring the constructs that are less relevant to the features of addiction, such as daily stress levels, impulsivity, and quality of life. Multiple regression analysis revealed that the IAT D2SD was independently and positively associated with smartphone addiction (p = .03) after controlling for other clinical correlates. Conclusions This study demonstrated good convergent and discriminant validity of this IAT as a novel measurement relating to Internet and smartphone addictions. Further longitudinal and prospective studies are needed to evaluate its potential utility in clinical and community settings.