This study aims to develop and assess the effectiveness of the HOTS Mathematical Problem Solving Framework (HMPSF) with the Bar Model Strategy on the achievement of Year Five Students. This study is a design and development study involving three phases. The first phase is a needs analysis study administered to 65 National School Mathematics teachers and 102 Year 5 students. The second phase is the design and development phase using the Fuzzy Delphi method involving 15 experts in Mathematics Education. Phase three is the implementation and testing phase of HMPSF with the Bar Model Strategy. In this phase, the study was conducted through a quasi-experimental design of a non-equivalent group of pre-post analysis involving 63 Year Five students from two schools in Kinta Utara district, Perak, Malaysia. The sampling process is done randomly. Data were analyzed through ANOVA and MANOVA. The needs analysis conducted show that 92.2% of teachers and 98.04% of students stated that there is a need to develop the HMPSF. Fuzzy Delphi analysis showed that the three main conditions were met (i) Threshold value, D ≤ 0.2, (ii) Percentage of expert agreements,> 75% and (iii) α-cut value (Fuzzy score), ≥ 0.5. This finding shows that the HMPSF with the Bar Model Strategy has been successfully developed through the agreement of 15 experts. The findings in phase three indicate that there is a significant difference in the mean of the post-group test for student achievement. The findings of the Multivariate Pillai’s Trace test also show that there is a significant effect between the pre and post-test. In conclusion, the HMPSF with the Bar Model Strategy developed is proven can enhance student achievement. Conclusively, the HMPSF with the Bar Model Strategy is suitable as a main guide for teachers and students in strengthening problem-solving skills in the field of Numbers and Mathematical Operations.