Teaching methods significantly influence learners’ academic achievement and motivation. This study investigated the integration of Inquiry-Based Learning (IBL) in the instruction of Physics and its influence on student achievement. In particular, the research explored the degree to which secondary school Teachers of Physics in Nairobi City County, Kenya, utilized IBL strategies, assess the impact of these strategies on student performance, and identify challenges teachers face in implementing IBL. The research was based on Constructivist Theory and utilized the Descriptive survey design to collect and analyze data. The research targeted a population of 3200 Form Two students taking physics and 75 teachers teaching physics in Nairobi City County Public Secondary Schools. Eleven schools were chosen using stratified random sampling technique. Purposive sampling technique was employed to pick 11 teachers while 326 students in Form two were selected for the study using simple random sampling technique. For data collection, the study used questionnaires, interview, and Lesson Observation Schedule. For pilot study, two schools participated in order to determine the validity and reliability of the research instruments. Data was collected using structured questionnaires administered to both students and teachers. Lessons were attended for Observation and Interview carried out with the teachers of physics. The study employed descriptive and inferential statistics through the use of SPSS to analyze the quantitative data. Findings from the study showed that the teachers of physics do not fully integrate IBL in their teaching of physics. The study also concluded that IBL has positive influences on learner achievement in physics and overall learning outcomes when fully utilized. The study also identified lack of enough resources, lack of enough laboratory apparatus, lack of professional training in IBL, lack of enough time to cover the syllabus as well as large class sizes as the main challenges to implementation of IBL in physics lessons. The study recommends that Continuous Teacher Professional Development training on IBL oriented pedagogy be enhanced, Integration of IBL modules in the training of teachers at the universities and colleges be enacted, allocation of sufficient funds to public secondary schools in order to have adequate teaching and learning resources by the government, alignment of the curriculum with IBL principles through incorporation of IBL activities in learning outcomes and lesson guidelines among others. The findings of the study inform the education stakeholders of the senior school science teachers’ preparedness in handling the Competency Based Curriculum starting in grade 10 in the new Competency Based Education system in which IBL is highly recommended as a teaching approach for the science subjects