This research was conducted in two studies with the aim of investigating the mental health of adolescent girls who have relationships with opposite sex. The first study used a qualitative approach and a phenomenological method, and the second study used a quantitative approach and a causal-comparative method. In the first study, 12 adolescent girls aged 12 to 18 years old from Amlash city in 2025 who had a regular, daily, and at least one-year emotional relationship with the opposite sex were selected through purposive sampling and subjected to semi-structured interviews. In the second study, 30 adolescent girls who were in a romantic relationship with the opposite sex were selected and compared with 30 girls who were not in such a relationship. The research instruments in the second study were the Ryff Scale Psychological Wellbeing–Short Form (RSPWB-SF; Ryff, 1995) and the Mental Health Questionnaire (GHQ-12; Goldberg, 1978). The coding of the interviews in the first study revealed 3 main themes and 27 sub-themes. In the second study, the findings showed that girls who have an emotional relationship with a opposite sex partner experience a significantly lower level of mental health and psychological well-being than girls who do not have such a relationship (P < 0.05). According to the findings, it can be concluded that adolescent girls have their own concerns, fantasies, and worries about emotional relationships with an opposite sex partner, which requires the attention of families, schools, and culture-building in society to prevent psychological harm and reduce their mental health.