Educational inequality is a highly debated yet empirically understudied topic in Malaysia. This paper examines the patterns and trends of academic achievement gaps by student social groups in Malaysia, drawing upon nationally representative data for the most recent four cohorts (1999, 2003, 2007, and 2011) of eighth-grade Malaysian students from the Trends in International Mathematics and Science Study. Results show that while the achievement gaps by gender (female vs. male) and ethnicity (Malay vs. non-Malay) existed at various time points and have changed over time, the magnitudes of change were small. Most alarmingly, the achievement gaps both in math and in science between students from high and low socioeconomic families have rapidly widened and became more prominent than gender and ethnicity gaps in recent years. If this trend continues, it will have dire consequences for economic equality and social stability in the society. To reduce educational achievement gaps, policymakers and educators should play a more active role in supporting academically disadvantaged students, especially those who are from underprivileged backgrounds.