Abstract: The school curriculum consists of formal, informal and non-formal dimensions. Secondary schools in Kenya, however, tend to emphasize formal curricular activities. Non-formal activities (NFA) are given less emphasis. Little was known regarding the contribution of non-formal curricular of science club activities (SCA) on students’ interest and achievement in Physics. In Vihiga County of Kenya, participation in SCA is poor with only 25 per cent rate of participation of schools. Yet students’ achievement in Physics in the County has been low with mean grade of D over the years 2006 to 2014. The purpose of this study was to determine the influence of SCA on secondary school students’ interest and achievement in Physics in Vihiga County.