This mathematics classroom based experimental study determined the relative effectiveness of formative assessment techniques on students' academic achievement in Mathematics. Factorial research design was adopted for the study involving 100 students from 2 co-educational secondary schools drawn through multistage sampling procedure. Mathematics Achievement Test was the instrument used for data collection. The reliability of the instrument was established using Kudder Richardson (KR[subscript 20]) formula and a reliability coefficient of 0.92 was obtained. Descriptive statistics of mean and standard deviation were used to answer the research questions and Analysis of Covariance (ANCOVA) was employed to test the hypotheses at 0.05 level of significance. The findings of the study revealed that the muddiest point formative assessment technique proved to be more effective in improving students' academic achievement in Mathematics than the think-pair-share technique. The findings also showed that the mean achievement scores of male and female students in Mathematics are not statistically significant. The muddiest point technique should be employed regularly by teachers for classroom assessment of the students since it is more effective in improving students' academic achievement in Mathematics.