Previous studies have shown that bullying and victimization can be experienced simultaneously by an individual and can change over time. Understanding the joint longitudinal development of the two is of great significance. We conducted a 4-year longitudinal study to examine the joint developmental trajectories of bullying and victimization, gender and grade differences in trajectory group membership, and changes in specific forms of bullying and victimization (verbal, relational, and physical bullying/ victimization) in each trajectory group. A total of 775 children from China participated in our study. The average age of participants at the first wave was 10.90 years (SD = 1.12), and boys accounted for 69.5% of the sample. Based on average scores, four distinct joint developmental trajectories of bullying and victimization were found: the involvement group (both bullying and victimization increased from low to high over time, accounting for 7.6% of the total), the desisted group (both bullying and victimization decreased from high to low over time, 6.1%), the victimization group (victimization remained at a high level, whereas bullying remained at a low level for 3 years, 13.2%), and the noninvolved group (bullying and victimization remained at a stable low level, 73.1%). Boys were more likely than girls to belong to the involvement group, desisted group, and victimization group, whereas girls were more likely than boys to belong to the noninvolved group. There was no significant grade difference in the trajectory group. All forms of bullying/victimization were consistent with the overall trend and showed similar levels. These results have important implications for the prevention of and interventions for school bullying.