The study explored how school principals' transformational leadership impacts formal and informal teacher collaboration for technology integration. Goal clarity and the strategic importance of technology integration in Swiss upper-secondary schools were considered as mediators. Analyzing data from a survey of 2247 teachers using multilevel correlation and structural equation modeling (SEM) with cluster-robust standard errors, we found that principals’ transformational leadership is significantly and positively correlated with both formal and informal teacher collaboration, goal clarity, and strategic importance of technology integration. Further, SEM analysis revealed that transformational leadership has a significant, positive, and direct effect on formal collaboration but has only a significant indirect effect on informal collaboration. Moreover, the strategic importance of technology integration mediates the relationship between transformational leadership and formal collaboration as well as informal collaboration. However, goal clarity was only a significant mediator for the relationship between transformational leadership and informal collaboration. These findings suggest that for effective technology integration, school principals should prioritize transformational leadership to ensure a high strategic importance and goal clarity regarding technology integration which in turn have a positive impact on formal and informal collaboration.