Smartphones have become the backbone of the connected society, reshaping social interactions in a period of adolescence marked by a neuropsychology vulnerability that is sensitive to intensive technological mediation. This study analyzes the relationship between the problematic use of mobile phones and the social and assertiveness skills of adolescents. Through a cross-cutting design, the answers of 1864 adolescents aged between 11 and 21 years old from education centers located in Cordoba (Spain) were analyzed, through a questionnaire that collected sociodemographic variables, the MPPUSA scale, to measure the inadequate use of mobile phones, and the ADCA-1 to assess social skills and assertiveness. The results revealed inadequate levels of mobile phone use and low levels of social skills, with nomophobia and negative consequences as the main risk factors, with the cluster analysis confirming the latter as the main predictor of the level of social development. The findings point to a concerning situation, in which not only does the usage time, but also the quality, have an influence on the psychosocial development of this population group. The application of preventive and educational interventions that address literacy, management of emotions, and the promotion of face-to-face social skills are therefore necessary.