The teaching profession is associated with high levels of perceived stress due to time constraints, heavy workload, and extra-curricular obligations. Teachers' perceived stress affects the quality of their instruction and consequently their students' motivation. According to social interdependence theory, frequent cooperative activities with colleagues may lead to more giving and receiving of support at the workplace. Research findings indicate that colleague support serves as a resource for teachers and has a positive influence on their performance. However, the relationship between teachers' perceived stress, goal-oriented cooperation with colleagues, and support of one another has been explored rarely. The theoretical background has not been applied to teachers in Germany. In this longitudinal study, 2,648 teachers completed the same questionnaire at a first measurement wave (Time 1), a second wave 2 years later (Time 2), and a third measurement wave 2 years after that (Time 3). We aimed to test the mediation hypothesis that teachers' perceived stress is affected by frequent cooperative lesson planning with colleagues via colleague support. The dependent variable was teachers' perceived stress at Time 3. Teachers' perceived stress correlated negatively with colleague support at Time 2, and this correlated positively with the frequency of cooperation at Time 1. Our results indicate that reduced perceived stress was indirectly associated with frequent cooperation in reaching the common goal of planning lessons via colleague support among teachers. These findings might be used to help school principals ensure cooperation among teachers and thereby keep stress levels low and the quality of teaching high.