The ISoLA 2024 proceedings constitutes contributions of the associated events held at the 12th International Symposium on Leveraging Applications of Formal Methods, ISoLA 2024, which took place in Crete, Greece, in October 2024. ISoLA 2024 provides a forum for developers, users, and researchers to discuss issues related to the adoption and use of rigorous tools and methods for the specification, analysis, verification, certification, construction, test, and maintenance of systems from the point of view of their different application domains.