In large-scale agile systems development, the lack of a unified requirements engineering (RE) process is a major challenge, exacerbated by the absence of high-level guiding principles for effective requirements management. To address this challenge, we conducted a five-year longitudinal case study with Grundfos AB, in collaboration with the Software Centre in Sweden. RE principles were first derived through qualitative data collection spanning more than 25 sprints, approximately 320 weekly synchronisation meetings, and seven cross-company/company-specific workshops between 2019 and 2024. These activities engaged practitioners from diverse roles, representing several hundred developers across domains. In late 2024, five in-depth focus groups with senior leaders at Grundfos provided retrospective validation of the principles and assessed their strategic impact. We aim to (1) empirically examine RE principles in large-scale agile system development, (2) explore their benefits in practice within the case company, and (3) identify a set of transferable RE principles for large-scale contexts. Using thematic analysis, six key RE principles—architectural context, stakeholder-driven validation and alignment, requirements evolution with lightweight documentation, delegated requirements management, organisational roles and responsibilities, and shared understanding of requirements. The study was further validated through cross-company expert evaluation with three additional multinational organisations (Bosch, Ericsson, and Volvo Cars), which are directly responsible for large-scale requirements management. Together, these efforts provide a scalable and adaptable foundation for improving requirements practices in large-scale agile organisations.