This study investigated metacognitive monitoring as a predictor of mathematics achievement among students in public secondary schools in Kisii Central Sub County, Kenya. The study was guided by the Social Development Theory (1978) by Lev Vygotsky and the Theory of Education Productivity by Walberg (1981). The study employed the Solomon Four pretest-posttest two group design with posttest only control design. The study population included 1665 form 3 students and 41 form 3 mathematics teachers from public secondary schools in Kisii Central Sub County, Kenya. Purposive, stratified and simple random sampling technique was used to select the participants. The sample size comprised of 360 form 3 students and 11 form 3 mathematics teachers. The findings revealed a statistically significant positive correlation between metacognitive monitoring and mathematics achievement. The study further established that students who monitored their performance did better in mathematics than their counterparts who did not monitor their performance. The study recommended guidance counsellors be trained to identify students with weak metacognitive monitoring skills so that they could be assisted to perform better in mathematics. Future studies could investigate the external environment as a predictor of mathematics achievement among students in secondary schools.