Prosocial behavior is actions taken or planned to help others, regardless of the actor's motives. Individuals who have high prosocial behavior will help others without expecting anything in return. However, prosocial values are declining. Therefore, action is needed to improve students' prosocial behavior, including by providing group guidance services. This study aims to determine the effectiveness of group guidance service using simulation game techniques to improve students' prosocial behavior in Grade VIII of State Junior High School 1 Bantul. This quantitative study used a pre-experimental design, one group pretest-posttest design model. The simulation game technique was carried out for six treatments. The subjects of this study were 10 students in grade VIII of State Junior High School 1 Bantul with a low category of prosocial behavior. The data were collected by using a prosocial behavior scale. The data were analyzed using paired sample t-test. Based on the results of the study, it can be seen that the group guidance service using simulation game techniques are effective in improving the students' prosocial behavior in Grade VIII of State Junior High School 1 Bantul.