The present study aimed to explore the effectiveness of group counseling of mindfulness-based cognitive therapy on internet addiction and cognitive emotion regulation in high school students. The research design was quasi-experimental and pretest-posttest type and a two-month follow-up with a control group. The population consisted of all male high school students in Takestan city in the academic year 2020-2021. The sample comprised 30 high school male students selected through convenience sampling method and randomly assigned to experimental (n = 15) and control (n = 15) groups. The experimental group then received 10 sessions of 90-minute group counseling of mindfulness-based cognitive therapy, and the control group did not receive any intervention. Data were gathered by Young’s Internet Addiction Questionnaire (IAQ) and the Cognitive Emotion Regulation Questionnaire (CERQ). The researcher also used multivariate analysis of covariance, repeated measures, LSD post hoc test and SPSS-26 software for data analysis. The results of comparing the experimental and control groups indicated that group counseling of mindfulness-based cognitive therapy had a significant effect on Internet addiction in high school students (P < 0.01). Furthermore, the results showed that Group counseling of mindfulness-based cognitive therapy had a significant effect on cognitive emotion regulation (P < 0.01). The findings indicated that group counseling of mindfulness-based cognitive therapy is an effective intervention in reducing Internet addiction and cognitive emotion regulation of secondary high school students.