Institutional Culture has a crucial influence on Strategic planning implementation of many educational institutions. Strategic planning has been employed in secondary schools as a transformative tool to re-engineer institutions of learning resulting into improved performance. School strategic plan can guide an organization to clarify further direction to identify its priorities, diversify its products and services and be ready to handle changing circumstances. Despite the efforts made by most secondary schools to implement the strategy, its desired outcomes have not been achieved due to the influence of various institutional culture, hence the need to conduct the current study. The study was targeted 150 Principals and 500 Heads of Departments. 75 Principals and 250 Heads of Departments were selected purposively using stratified probability sampling technique. Reliability of the questionnaire was established by the test- retested try-out which was conducted in non-participation institutions. Validity of the instruments was ensured by presenting the instruments to three experts from the Department of Educational Management and Policy Studies of Moi University who examined the content of the instruments and advised the researcher on the face validity. Improvements were made according to the suggestions made by the experts before the instruments were finally taken into the field. The study found out that Institutional culture influences strategic planning implementation in secondary schools. The study recommends that, Secondary schools should establish cultural practices depending on the location of the institutions, history of the institutions and the dynamic nature of the environment in which they operate.