The aim of this research is to describe the impact of implementing an integrated STEAM Project via mobile technology on the Mathematical Reasoning ability of elementary school students and to describe the process of designing an integrated STEAM Project and mobile application. A quantitative descriptive and quasi-experimental research design was used, with data collected by observation. A pre-test, and post-test was administered to determine the impact of the STEAM Project on students' reasoning abilities. A sample of 130 fifth grade students, 70 female students, and 60 male students from different elementary schools in Meuraxa District, Aceh Province, Indonesia participated in the research. Results show that an empirical analysis of mobile technology is valid and feasible for use with fifth grade elementary school students. Prior to implementing the STEAM Project, the average pretest of reasoning ability was 40.44 with a standard deviation of 10.27. After implementation, the average score improved to 84.01 with a standard deviation of 7.31. The results of a Paired Sample Correlation obtained a mathematical reasoning ability score before and after implementation of the STEAM Project of 0.452. The calculations in the N-GAIN test table for the implementation of the STEAM Project on Reasoning Ability found 76 students, or 58.46 %, in the high category, and 54 students, or 41.54%, in the medium category. The results of this study provide information, and recommendations for the development of learning models that could improve reasoning abilities. The project could be used as a reference template to accelerate the recovery of quality education during challenging times such as a pandemic.