This study determined the effectiveness of ethnomathematics teaching approach, ETA on students' achievement and retention in Locus. The study was carried out in education zone B of Benue State of Nigeria using a sample size of 253 Senior Secondary 2 (SS 2) students. It was a non equivalent quasi-experimental study which was guided by two research questions and two hypotheses. Locus Achievement Test (LAT) instrument with a reliability coefficient of 0.78 was used as pre, post and delayed tests though reshuffled each time. Answers to the research questions were given using mean and standard deviation while the hypotheses were tested at 0.05 significant level using a 2-way analysis of covariance (ANCOVA). Results from the analysis revealed that students exposed to ETA were superior in achievement and retention than those taught with conventional approach. Thus there were significant differences between the mean score of the students taught Locus with ETA and those taught with the conventional approach in both achievement (F[subscript 1, 248] = 241.317, p = 0.000) and retention (F[subscript 1, 248] = 270.421, p = 0.000). The study therefore recommended training of mathematics teachers on the use of ethnomathematics in their daily lessons especially in Locus with a view to making learning meaningful, relevant and interesting. (Contains 4 tables.)