This study was carried out to investigate the impact of staff development programmes on teachers’ effectiveness in Ekiti state secondary schools. It sought to find out the influence that development programmes such as seminars, conference, symposium, workshops, and further studies could have on teachers’ professional effectiveness in areas such as mastery of subject matter, planning, classroom management, self-management and relational ability, and communication. A descriptive research design was employed and the population described include all the teachers in Ekiti state secondary schools. A total of 20 public secondary schools were randomly selected and 200 teachers who had previously been exposed to development programmes were purposively and proportionately selected from these 20 secondary schools. A self-designed questionnaire titled ‘Staff Development Programmes and Teachers Effectiveness Questionnaire’ was adjudged to valid by experts in Educational Management and found to be consistent after been subjected to reliability test using the test-retest method with Spearman rank order correlation coefficient of 0.80 at 0.05 level of significance, was used. The questionnaire was administered personally by the researcher. The data collected were analysed with the use of frequency counts, chi-square, and t-test. The study shows that development programmes influence teachers effectiveness in the school system. It was revealed that development programmes significantly influence teachers’ mastery of subject matters, planning ability, classroom management ability, self-management and relational ability, and communication skill in the state secondary schools. It is recommended that teachers development programmes should be provided on a continual basis for all categories of teachers and that such programme should focus on the enhancement of mastery of subject matters, planning, classroom management, self-management and relational skills, and communication.