The study examined the influence of language diversity on organizational communication and effectiveness in secondary schools in Lagos State, Nigeria. The study was guided by two research questions and two hypotheses. The correlational research method was used in the study. The population for the study comprised 19,978 public secondary school teachers in Lagos State for the 2023/2024 session. The sample size for this study comprised 672 teachers selected from public secondary schools in Lagos State using stratified random sampling technique. Questionnaire was used for data collection. The questionnaire was duly validated by three experts had a reliability coefficient of 0.77. Data obtained with the aid of the questionnaire were analysed using Pearson correlation. The results revealed: a significant negative relationship between language diversity among staff and students in secondary schools in Lagos State and organizational communication; a significant negative relationship between language diversity among staff and students in secondary schools in Lagos State and organizational effectiveness. Based on these findings, it was concluded that increased language diversity among staff and students can hinder effective communication, leading to challenges in collaboration, understanding, and the overall functioning of educational institutions. It was therefore, recommended amongst others that secondary schools should establish language support initiatives, such as language workshops or tutoring programs, aimed at enhancing the language proficiency of both staff and students.