Indonesian students' achievement in mathematics is generally still low compared with other countries. Many psychological factors, both internal and external, influence this poor performance. This study aimed to measure the effect of self-efficacy and the adversity quotient of Grade IX students regarding achievement in mathematics. Both of these internal variables have been selected because students' success in mathematics is determined more by internal factors than by external factors. A survey method was used. The sample included 140 students and was drawn using a probability sampling technique. A self-efficacy scale and an adversity quotient scale were used to collect the data. Students' mathematics achievement was determined based on school test results. The data were analyzed using multiple regressions. The findings reveal significant effects of self-efficacy and the adversity quotient but no significant effects of gender on students' academic mathematics achievement. Therefore, an implication of the study is that we must investigate how to improve students' self-efficacy and adversity quotient in mathematics. The results may be of interest to other developing countries, especially those in Southeast Asia that share similar concerns with Indonesia regarding students' mathematics achievement.