This study investigated the impact of in-service training and staff development on workers' job performance and optimal productivity in public secondary schools in Osun State, Nigeria. The study used the ex-post-facto research design. Three research questions and three hypotheses were generated and tested using questionnaire items adapted from Raja et al. (2014) but subjected to manipulation by the researchers which contained closed ended type of questionnaire based on the research questions and hypotheses and was structured on a four point Likert scale. The instrument was administered to a purposely selected population of 152 respondents while 134 questionnaires were returned. Data generated were analyzed using Analysis of Variance (ANOVA) and Multiple Regression Analysis to test the hypotheses at 0.05 level of significance. The findings show that results showed that in-service training and staff development had insignificant combined effects but significant relative effects on workers' optimal job productivity. The study therefore recommended that schools should design proper and functioning in-service training and staff development programmes for their workers to boost their morale, enhance their performance, and in addition ensure that workers' trainings are conducted frequently to ensure they cope with changing technological environment and organizational climate in schools.