Abstract This mixed-method research aimed to determine the effectiveness of teaching through problem solving to the mathematics performance of Grade 2 pupils and to describe the heuristics employed by the pupils in answering the mathematical problems. The quantitative part of the study employed the pretest-posttest quasi experimental research design while the qualitative inquiry involved case study of multiple cases. The result of the pretest and posttest were used to determine the effect of teaching through problem solving through the use of mean, standard deviation, t test for independent samples, and t test for paired samples. For the qualitative part, data gathered with the use of audio recordings of the interview were transcribed, analyzed, and interpreted through thematic analysis. Results revealed that both groups significantly increased their scores; hence, both lecture-discussion and teaching through problem solving have improved the pupils’ performance. However, teaching through problem solving is a better teaching strategy since the posttests in both groups are significantly different. The result of the posttest scores revealed that the teaching through problem solving group’s mean score (M = 35.54, SD = 8.99) was higher than the mean posttest score of the lecturediscussion group (M = 17.07, SD = 7.78). This difference was found to be significant, t (25) = 13.19, p =.000. This implies that teaching through problem solving is more effective than lecture-discussion. Four heuristics in answering the problem solving emerged in the thematic analysis: (1) direct computation; (2) use of drawings; (3) use of symbols; and (4) use of guess and check. Based on the results of the study, the use of teaching through problem solving as an approach in teaching mathematics is effective in improving the performance of Grade 2 pupils. This study implies that teaching through problem solving is effective in improving the mathematics performance of Grade 2 pupils and developing the heuristics of the pupils. It is highly recommended that Mathematics teachers should encourage pupils to become problem solvers, apply teaching through problem solving, and put into practice situated learning and constructivism.