This study investigates the relationship between principles-based project management methodologies (PMMs) and project success, with a specific focus on the Half Double methodology. Unlike traditional process-based approaches, principles-based PMMs—such as PMBOK and PRINCE2—emphasize guiding principles over rigid procedures, offering a more flexible and adaptive framework for managing projects. Despite their increasing adoption across industries, the empirical evidence linking these methodologies to successful project outcomes remains limited. To address this gap, a cross-industry, multi-informant, multi-level survey was conducted, encompassing 351 projects nested within 92 project port-folios. The findings reveal that three core principles of the Half Double methodology are significantly and positively associated with project success. This research contributes to the growing body of literature on principles-based project management by providing empirical support for the effectiveness of such methodologies. Additionally, it extends the semantic hierarchy of PMMs to explicitly include principles, and discusses the relevance, optimal number, and adaptability of these principles in dynamic organizational environments. The study underscores the importance of aligning PMMs with specific project contexts and characteristics, offering practical insights for managers seeking to enhance performance through more principle-driven approaches. Overall, the findings highlight the strategic value of adopting principles-based methodologies in achieving successful project outcomes.