Abstract: Teacher professional development support by principals played a key role in ensuring improved academic performance in schools. However, in Gatanga Sub-county, many public secondary schools register low academic grades in KCSE. Thus, this study examined the influence of teacher professional development support on students’ academic performance in public secondary schools in Gatanga Sub-county. The study was guided by professional development theory and academic achievement theory. The study adopted a mixed methodology and, thus, applied a concurrent triangulation research design. The target population was 597 respondents, which comprised 35 principals, 560 teachers, and 2 Sub-county Directors of Education (TSC & MoE), from which a sample of 242 respondents was determined using Yamane’s Formula. This included 15 principals, 225 teachers, and 2 Sub-county Directors of Education (TSC & MoE). Qualitative data were analyzed thematically based on the objectives and presented in narrative form. Quantitative data were analyzed descriptively using frequencies and percentages and inferentially using Pearson’s Product Moment Correlation Analysis using the Statistical Package for Social Sciences (SPSS Version 25) and presented using tables. The study found that students’ academic performance in KCSE in public secondary schools has been low for the last five years (2020-2024). This has been partly attributed to the fact that principals rarely provide adequate support for teachers’ professional development, a situation that has significantly contributed to low academic performance among students. Thus, the Ministry of Education should allocate resources to enable principals organize regular retraining and mentorship as well as coaching programmes for teachers.