ABSTRACT : Academic achievement in Integrated Science among Junior School learners in Bomet County serves as the main dependent variable of this study, set against a backdrop of persistent underperformance that jeopardizes Kenya Vision 2030’s goal of transitioning 60% of learners into Senior School STEM pathways. Diagnostic baseline data revealed that over 60% of learners in the 2023 KPSEA, Grade Seven and Eight SchoolBased Assessments, and 2025 KJSEA performed at or below “Approaching Expectations,” mastering only 30% to 40% of key scientific skills. To address this performance gap, the study evaluated four Total Quality Management (TQM) dimensions, focusing on Teacher Professional Development (TPD) as the primary independent predictor of academic achievement. Guided by a pragmatist philosophy, the study utilized a mixedmethods concurrent triangulation design. From a target population of 21,653 individuals across Bomet County,comprising 5 Sub-County Directors of Education (SCDEs), 541 Grade Nine parents’ representatives, 541 Heads of Institutions (HOIs), 541 Integrated Science Facilitators (ISFs), and 20,025 Grade Nine learners—a total sample of 3,050 participants was involved. Yamane’s formula and proportionate stratified random sampling were used to select a quantitative sample of 230 HOIs (N = 194 valid responses returned), while qualitative data involved 5 SCDEs, 23 parents’ representatives, and 36 classroom observations encompassing 36 ISFs and 2,756 Grade Nine learners.