This article explicates the conception and evaluation of an information and communications technologies (ICT) professional development process for developing teachers' technological pedagogical content knowledge for 21st century learning. The process emphasizes teachers' prolonged engagement with peers and researchers in design teams. Supported by technological pedagogical content knowledge rubrics and technological pedagogical content knowledge design heuristics, the process enabled teachers to assess their current ICT lessons, set design goals, redesign, implement, and evaluate student learning outcomes, as well as reflect on their pedagogical practices. A year-long implementation study conducted with 37 teachers from a Singapore primary school who were organized into seven lesson design teams found that the process had positive effects on teachers' confidence for technological pedagogical content knowledge for 21st century learning and lesson design practices. Five of the seven design teams were able to make pedagogical changes toward 21st century learning, and six of the teams realized improvement in student learning outcomes. The implications for teacher ICT professional development are discussed.