The study investigated the potency of ethnoscience instruction on the critical thinking ability of biology students. The study employed a quasi-experimental research design, with a total of 113 senior secondary school two (the equivalent of grade 11) students from two schools in educational district V of Lagos State, Nigeria. All students in the two sample schools used for the experimental and control groups took a pretest before treatment, a posttest at the end of the treatment phase. Test of Critical Thinking in Photosynthesis (TCTP) with a reliability of 0.76 was used for data collection. The ANCOVA output demonstrated a statistically significant difference in favor of the experimental group (F (1, 110) = 14.79; p < 0.05). Ethnoscience Instruction had no differential impact on students in the experimental group based on gender. We concluded that ethnoscience instruction is a viable culturally relevant tool for improving critical thinking skill in biology.