Population aging and the increasing digitalization of financial services pose significant challenges to the inclusion of older adults, who often face significant usability and security barriers on Internet Banking (IB) platforms. Our main objective was to develop and validate the GABI Guide (A Welcoming Banking Guide for Older Adults), a compendium of actionable design guidelines for creating intuitive and secure IB interfaces. We used a mixed-methods approach: a qualitative diagnosis through interviews with 12 older adults identified core difficulties (e.g., fear of errors), while a concurrent Systematic Literature Review (SLR) established the theoretical foundation (selecting 13 articles from 2,378). This data triangulation culminated in the GABI Guide and its development as an interactive web platform (functional artifact). Results showed GABI’s exceptional rigor: the platform achieved a perfect Lighthouse Accessibility score (100). Furthermore, validation with 14 technology professionals confirmed the guidelines’ clarity and high relevance with an average score of 4.93/5.0. This work delivers a practical, validated, and socio-technical resource that translates ethical needs into engineering criteria, with the potential to significantly promote autonomy, security, and digital financial inclusion for older adults.