Globally girls' performance in mathematics has improved over the years as measured by TIMSS results. Inconsistency in mathematics performance of boys and girls is still an area of great concern. This study examines the relationship between mathematics achievement and gender of South African Grade 9 learners from TIMSS 2015 data. The data were further analysed to highlight any statistical relationships between gender and mathematics achievement using SPSS version 27. The results show that gender differences did not impact the overall learner achievement in most of the questions in the four content domains: algebra, data and chance, geometry, and numbers. However, the findings indicate that girls performed better in the questions that show a statistically significant gender difference. Yet, the findings show that boys tended to perform better than girls on questions that require reasoning compared with knowing and applying. The study highlighted that statistically significant gender differences varied by content and cognitive domains. Further research is needed to determine the reasons for these variations.