The present study was undertaken to design a parent-adolescent psychological intervention to reduce loneliness based on the components of loneliness of teenage girls and the evolutionary theory of loneliness of Cacioppo as well as conducting a case study on loneliness and parent-child interaction. The research had a single-subject design (AB multiple baseline design). The statistical population of the study was comprised of adolescent girls aged 16 to 18 along with their parents who were selected through purposive and voluntary sampling methods from two schools. The sample included two students along with their parents. The UCLA Loneliness Scale was used to measure loneliness and the Parent-Child Interaction Questionnaire (PACHIQ) was utilized to measure parent-child interaction. Based on the results, a psychological intervention consisting of 11 sessions was designed and implemented for parents and adolescents. The quasi-experimental case validation method (single subject) was used in the next stage. More specifically, loneliness, the parent-child interaction from teenagers’ point of view as well as the parent-child interaction from their parents’ viewpoint were examined and tested using the multi-level AB design in fourteen weeks. The results indicated that the developed psychological intervention reduced loneliness in female students and improved parent-child interaction from both adolescents and parents’ perspectives.