This study examined the relationship between learning environment and effectiveness of secondary schools in Nigeria. The descriptive research design of the survey type was used for the study. The population consisted of all the teachers and principals in the public secondary schools in south-west Nigeria. The sample comprised 1200 teachers and 60 principals from 60 secondary schools. Multi-stage, simple and stratified random sampling techniques were used to select the states, schools and teachers used for the study. Two sets of instrument were used to collect the data for the study. The data were analysed using frequency counts, percentage scores and Pearson product moment correlation. The hypothesis was tested at 0.05 level of significance. The study revealed that the learning environment in secondary schools was very conducive and the secondary schools were effective in the affective and the psychomotor domains of learning but not all that effective in the cognitive domain. The study further showed a significant relationship between learning environment and effectiveness of secondary schools. Based on the findings, it was recommended that the school administrators and other stakeholders in the secondary education sector should strive hard to sustain the tempo of the conducive environment for learning. It was also recommended that schools should sustain the tempo of achievements of the students in the affective and the psychomotor domains of learning. Efforts should also be made at improving upon the level of cognitive achievement of the learners, as this will enhance the improvement on the general level of school effectiveness.