The study investigated teachers’ understanding of student discipline in public secondary schools in Oyo State, Nigeria. It examined the public secondary school teachers’ knowledge of student discipline in the 21st century. It further determined the appropriateness of strategies adopted by teachers in enforcing discipline in schools and as well examined the challenges facing the enforcement of discipline in public secondary schools. The study adopted a survey design. The population of the study comprised all the public secondary school teachers in the Oyo central senatorial district of the state. The sample size comprised 200 senior secondary school teachers selected across public schools in the Oyo East Local Government Area of the state using a simple random sampling technique. A questionnaire titled Discipline Knowledge and Strategies Assessment Questionnaire (DKSAQ) was used to collect data for the study. Percentage, mean, standard deviation, and ranking were used to analyse the data. The results showed that public secondary school teachers had incomplete knowledge about student discipline in the 21st century. The results showed that secondary school teachers adopted both appropriate and non-appropriate discipline strategies. Students’ unfriendly home environments, parents’ inability to provide basic needs, and fear of reprisal from students or their gangs were some of the identified factors militating against the enforcement of discipline in schools. The study, therefore, concludes that public secondary school teachers’ knowledge of school discipline in the 21st century is inadequate and recommends that modalities through which teachers can have access to new knowledge and skills be put in place by their employers.