Principals’ leadership characteristics cannot be underrated in enhancing teachers’ effectiveness. This study investigated the influence of principals’ leadership characteristics on teachers’ effectiveness in Southwestern Nigerian secondary schools. The study adopted a quantitative approach with a survey research design. The population comprised all 2,596 principals and 53,836 teachers in all 2,596 public secondary schools in Southwestern Nigerian. The sample of this study consisted of 972 respondents that comprised 54 principals and 918 teachers. Two instruments were used and they were: (a) Teachers’ Effectiveness Questionnaire (TEQ); and (b) Personnel Leadership Characteristics Questionnaires (PLCQ). Data collected were analyzed using descriptive and inferential statistics. The outcomes of the study showed that teachers in Southwestern Nigerian public secondary schools were moderately effective. The study further found out that integrity, justice, humanity, courage, humility, collaboration, accountability, and temperance respectively were leadership characteristics exhibited and demonstrated by principals in Southwestern Nigerian public secondary schools. Furthermore, the results indicated that integrity, humanity, courage, humility, collaboration, justice, accountability, and temperance were significant contributors to teachers’ effectiveness in Southwestern Nigerian public secondary schools. The study, therefore, concluded that those principals’ leadership characteristics (integrity, justice, humanity, courage, humility, collaboration, accountability, and temperance) positively influenced teachers’ effectiveness in Southwestern Nigerian public secondary schools. The study implicates that principals of schools are expected to develop and exhibit the leadership characteristics that are consistent and acceptable in situations that would contribute, trigger, and enhance the teachers’ effectiveness toward the achievement of educational goals.