One of the best ways to develop students' critical thinking abilities is to incorporate STEM (Science, Technology, Engineering, and Mathematics) disciplines with Project-Based Learning (PjBL). This study emphasises the originality of integrating STEM with PjBL to explicitly improve critical thinking inside thematic learning environments, even though PjBL is recognised to aid cognitive development and problem-solving. This study examined the effects of STEM-PjBL on elementary students' critical thinking during science-themed learning. The study sought to demonstrate how STEM-PjBL, as opposed to regular PjBL, promotes deeper critical thought, analysis, and problem-solving, building earlier findings. 24 fifth-grade students from SDI Surya Buana Malang participated in the study, which used a pre-experimental design with a one-group pre-test-post-test methodology. Five essay questions that were verified by specialists and proved to be reliable (Cronbach's alpha = 0.805) were used to evaluate the students' critical thinking abilities. The results show that, in this particular setting, students' critical thinking abilities significantly improved after the STEM-PjBL intervention. These results indicate the model's potential for broader educational applications beyond cognitive benefits and show how well it can foster critical 21st-century abilities. Future studies could examine how this paradigm affects abilities other than critical thinking.