Objectives: Disorders of amino acid metabolism fall under the category of inborn errors of metabolism that can be managed with a protein-restricted diet. However, adherence to such a diet often poses challenges, leading to low treatment engagement. Consequently, there is a pressing need for new resources to aid in dietary self-monitoring. The goal is to develop and implement “AminoApp,” an application tailored for dietary self-monitoring in patients with inborn errors of metabolism who are on a low-protein diet. Methods: The design and development of the application adhered to the user-centered design method. This approach emphasizes active participation and collaboration between users and designers/researchers throughout all stages of product development, including requirement gathering, prototype development, and evaluation. Usability was evaluated using the System Usability Scale, which has been validated in Portuguese. Results: The application’s features include a food diary, a food consultation area, exam records, a recipe calculator, and reports on diet composition and metabolic control. The usability test included four patients on a low-protein diet, three caregivers, and three healthcare professionals. The average usability score was 84.9, with averages of 77.5 for patients, 85.8 for caregivers, and 91.6 for professionals, indicating that the application is user-friendly. Conclusions AminoApp is the first application developed in Brazil designed to assist in managing inborn errors of metabolism that require a protein-restricted diet. It was found to be easy to use, and the initial results are promising. Further research is necessary to evaluate the impact of the application on metabolic control and treatment adherence.