The impact of religious organizations on curriculum implementation in Kenya is perceived differently by stakeholders. The first secondary schools in colonial Kenya were established by Christian missionaries. Their participation has been viewed by stakeholders as either positive or negative. The objective of the study was to assess the impact of religious organizations on implementation of curriculum in public secondary schools in former Western province of Kenya. The study was conducted in Bungoma, Busia, Kakamega and Vihiga counties of former Western province of Kenya. The study is based on Ludwig Von Bertalanffy Theory of General Systems from which the Systems Approach in the learning process is derived. The study adopted a descriptive survey design. The target population included: four County Directors of Education, twenty two Sub-County Education Officers, principals and teachers in secondary schools, religious leaders and students. Of the twenty two (22) sub-counties, the researcher used purposive sampling to carry out the study in ten sub-counties. This gave a total of 621 students, 120 teachers, 60 principals of secondary schools, 5 Islamic leaders, 10 church leaders, 2 County Directors of Education, 4 Sub- county Education Officers and 1 curriculum developer. This gave a sample size of 823 respondents. The instruments for data collection were questionnaires for principals, teachers, students and interview schedule for religious leaders as well as Education Officers. Focus Group Discussion and Observation Schedule were also used. To establish validity and reliability, the instruments for data collection were assessed by subject experts from the Department of Curriculum and Instruction. The questionnaires were also piloted and the anomalies were corrected before the researcher went to the field. Presentation and analysis of data was based on the objective of the study and was put in form of tables, bar-graphs, pie-charts and descriptions. The findings were that religious organizations have an impact on curriculum implementation in secondary schools. Sadly some of the religious organizations cause divisions in the schools they sponsor by recommending the transfer of teachers as well as antagonizing the principals who do not profess the same faith as the sponsor. From the above findings the conclusions made were that religious organizations have an impact on curriculum implementation in schools as they complement efforts of the Government of Kenya in provision of education. The policy framework given to religious organizations is not strong enough to take care of curriculum needs in the present day school environment. It was therefore recommended that religious organizations revisit their initial role in matters curriculum. There should be clear policy guidelines on the impact of religious organizations on curriculum implementation in secondary schools which should go beyond spiritual nourishment, guidance and counseling and teaching of Religious Education in schools in public secondary schools in Kenya.