This study set out to examine the impact of assertiveness training on the level of academic motivation and Internet addiction of high school students in Tehran. This quasi-experimental research had a pretest-posttest control group design. The statistical population of the study was comprised of all Tehrani 11th graders in school year 2017-18.  The drawn sample (N=80) was selected by multi-stage cluster sampling method and was assigned to two experimental (N=40) and two control groups (N=40). The instruments included the Academic Motivation Questionnaire (Abdekhoda’ee et al., 2008) and the Internet Addiction Test (Young, 1998). The experimental groups were given eight two-hour sessions of assertiveness training, while the control group received no intervention. At the end of the training, academic motivation and Internet addiction were measured during posttest phase and the data were analyzed via ANCOVA. The results indicated that the assertiveness training significantly reduced Internet addiction in students and it improved their academic motivation. Consequently, assertiveness training could be utilized as an effective method at schools.