This study aimed to investigate the impact of a yearlong, online teacher professional development (oTPD) program, Prime Online, on teachers’ mathematics knowledge for teaching (MKT) and to examine the components of a PD program that impacted participants’ MKT. The analysis indicates latent growth modeling and focus group data indicated growth in participants’ content knowledge and initial growth in knowledge of students from pretest to midtest, with a decline at the end of the program. The authors conclude that that Prime Online has potential to provide teachers with ongoing, rigorous, high-quality learning opportunities for impacting their knowledge of mathematics content and pedagogy and of their students.