Competency-Based Education (CBE) is a student-centered approach to teaching and learning that has been implemented in Kenya since 2019. It is now applied across all fields of study, including science, which emphasizes different competencies compared to the arts. This study sought to assess the preparedness of lower primary schools in Nairobi City County for the implementation of the competency-based science curriculum. The specific objectives were to evaluate the extent of school readiness, examine the level of curriculum implementation, and identify challenges hindering the process. The study adopted a descriptive survey design and was guided by Jerome Bruner's Constructivist Theory. The target population comprised 124 primary schools, including 424 science teachers and 124 head teachers in Westlands Sub-County. Respondents were proportionately drawn from the five wards, with 12% sampled from each. Data were collected using structured questionnaires based on a five-point Likert scale. A pilot study was conducted to validate the research instruments and ensure reliability. Data analysis employed descriptive statistics, including frequencies and percentages, using SPSS version 27.0. Findings were presented in tables and graphs for clarity. The results revealed that school preparedness had a positive influence on CBE implementation, while challenges demonstrated a negative correlation with the dependent variable. Specifically, correlation analysis showed negative associations between CBE implementation and school readiness (r = –0.375), factors influencing preparedness (r = –0.272), and implementation levels (r = –0.032). A positive relationship (r = 0.251) was observed only for challenges affecting implementation. Regression coefficients were 0.035 for school preparedness, 0.016 for influencing factors, 0.020 for implementation levels, and –0.353 for challenges, indicating that obstacles significantly hinder implementation. Furthermore, the study concluded that the implementation of the CBE science learning domain plays a vital role in improving learner competencies, yet its success is constrained by inadequate teaching materials, insufficient infrastructure, and poorly equipped science laboratories. Furthermore, findings highlighted a gap in teacher training, with many science educators lacking the necessary skills to align effectively with CBE requirements. The study recommends increased allocation of resources to strengthen infrastructural capacity in schools, with particular emphasis on equipping science laboratories for hands-on learning. Additionally, quarterly professional development seminars for science teachers should be organized to facilitate knowledge sharing, enhance pedagogical practices, and promote smooth and sustainable implementation of the competency-based science curriculum.