Novice elementary school teachers often experience deficiencies in pedagogical content knowledge (PCK) during their early years of professional practice. This study investigated the effectiveness of structured online mentoring, facilitated by subject-group leaders, on the perceived and enacted dimensions of novice elementary teachers’ PCK. The study employed a quasi-experimental pretest–posttest control-group design. A total of 148 novice teachers from public elementary schools in Urmia were selected through stratified cluster sampling and randomly assigned to experimental and control groups. Over a 10-week period, the experimental group participated in weekly 90-minute online sessions via the Skyroom platform, facilitated by 12 trained subject-group leaders. The instruments included a perceived PCK questionnaire, situational vignettes, and a rubric for assessing PCK in lesson plans. Descriptive statistics, including means and standard deviations, were used to summarize the data. Analysis of covariance and one-sample t-tests were used to examine the effects of the intervention, while correlation and regression analyses were conducted to explore the relationships between perceived and performance-based PCK indicators. The findings indicated that the intervention significantly improved perceived PCK, performance on situational vignettes, and lesson-plan quality. In addition, perceived PCK was positively, though moderately, associated with performance-based indicators, and teaching experience played a moderating role. The results suggest that online mentoring can serve as an innovative and effective approach to supporting the professional development of novice teachers in Urmia.