It is known that teacher noticing skills improve through different interventions such as video clubs, lesson study, and short-term professional development programs. However, it is not known whether this improvement is permanent and whether teachers can transfer their noticing skills into the classroom. It is extremely important to provide an enduring change in teacher noticing skills. Rather than short-term programs, implementing long-running professional development programs, which last almost 2 years, enables teachers to maintain their noticing skills. At this point, the current study aims to develop in-service middle school mathematics teachers' professional noticing of students' mathematical thinking on pattern generalization during their involvement in a 2-year online professional development program enriched with collaborative discussion. Accordingly, the study was built on the professional noticing of children's mathematical thinking framework, including three noticing skills: attending, interpreting, and deciding how to respond. Participants comprised 31 in-service middle school mathematics teachers with up to 15 years of professional experience working in public schools in seven different provinces of Türkiye. The teachers participated in the online PDP, including scenarios involving student strategies of particular mathematics content. The analysis of the data gathered from the pre-test and post-test let us conclude that the three noticing skills of the in-service middle school teachers exhibited incredible progress through their involvement in collaborative discussion in a long-running online PDP. Based on this result, the characteristics of the online PDP are put forth to develop teachers' noticing skills.