This study examined the effects of the Ethno-mathematics instructional approach and Problem-based learning strategy on students' interest, achievement, and retention in geometry in Benue State, Nigeria. Quasi-experimental design involving pretest, posttest, and post-posttest with two experimental groups and one control group. Twelve research questions guided the study and twelve hypotheses were tested at 0.05 level of significance. The population comprised 20,213 Junior Secondary Two (JS II) students, with a sample of 1,200 students selected using simple random sampling. Two instruments were used for data collection namely; Geometry Interest Ratings Scale and Geometry Achievement Test. Content validity index for GIRS was 0.78 and construct validity for GAT was 0.88. The reliability of GIRS was tested using Cronbach Alpha formula which yielded an index of 0.81 and K-R21 was used to determine the reliability index of GAT which yielded an index of 0.75. Data were analyzed using mean and standard deviation for research questions and Analysis of Covariance (ANCOVA) for hypotheses The findings indicated a significant difference in the mean interest ratings of students taught geometry using the ethno-mathematics instructional approach, problem-based learning strategy, and conventional teaching approach. However, no significant difference was found in the mean interest scores between male and female students taught geometry using the ethno-mathematics instructional approach or problem-based learning strategy. Additionally, there was a significant difference in the mean achievement and retention scores of students taught geometry using these approaches compared to the conventional teaching approach. No significant difference was found in the mean achievement or retention scores between male and female students taught using these approaches. The ANCOVA result on the interaction effect between methods and gender on retention indicates that there is no significant interaction effect between Ethno-mathematics instructional approach, Problem-based learning strategy and gender on retention. Based on the findings of this study, it is recommended that: Students should be subjected to consistent utilisation of ethno-mathematical operations within their culture, adoption of the ethno-mathematics instructional approach in the school system, and training of mathematics teachers in the use of ethno-mathematics instructional approach to improve students' interest, achievement and retention in geometry.