This study was conducted to develop and validate the instructional material for problem solving in Mathematics for Grade 4 pupils in an elementary school at the District of Tangalan, SY 2023-2024. The design-based research was used in this study using IPO model. The participants were the 66 Grade 4 pupils and the 10 teacher-experts. Reading comprehension test and learning material evaluation tool were the data gathering instrument. The data were analyzed and interpreted using descriptive and inferential data analysis. The least learned competencies in mathematical problem solving of Grade 4 pupils were: solving routine and non-routine problems involving multiplication of whole numbers including money using appropriate problem-solving strategies and tools, solving multi-step routine and non-routine problems involving multiplication and addition or subtraction using appropriate problem-solving strategies and tools and solving multi-step routine and non-routine problems involving division and any of the other operations of whole numbers including money using appropriate problem-solving strategies and tools. A Learning Activity Sheet was created based on the result of the study. The problem-solving skills in the pre-test were low while the post-test was moderate. There is a significant difference on the problem solving competence of Grade 4 pupils before and after the intervention using the learning activity sheets. The level of acceptability of the developed instructional material for grade 4 was very highly acceptable as to content, instructional quality, prints, design and layouts.