###### Abstract  
Urban stormwater management systems are increasingly strained by rapid urbanization and climate change, yet existing planning approaches often lack holistic optimization frameworks that account for both green and grey infrastructure (GREI) under uncertain future conditions. This study introduces a multi-objective optimization framework for Grey-Green Infrastructure (GGI), which integrates green infrastructure (GI) with GREI to enhance urban flood resilience, cost efficiency, and adaptability. The framework addresses life cycle cost (LCC), technological resilience (Tech-R), and operational resilience (Oper-R), offering a comprehensive approach to navigating the complexities of urban stormwater management. Key findings reveal that: (1) GGI systems optimized for resilience achieve a 33% improvement in Oper-R, with only a marginal increase in LCC of less than 9%, highlighting their robustness under GREI failure scenarios; (2) the integration of bioretention cells (BCs) and porous pavements (PPs) into GGI increases Tech-R by 7.1%, enhancing soil water retention and permeability, particularly in densely urbanized contexts; and (3) decentralized GGI systems exhibit superior adaptability to extreme weather events, with Design D reducing LCC to USD 53.9 M while maintaining no overflow under a 5-year rainfall event. The framework was validated in Zhujiang New Town, Guangzhou, where optimized GGI designs reduced average pipe diameters and manhole depths by 0.2-0.3 m compared to GREI-only systems, demonstrating both cost and resilience advantages. These findings provide decision-makers with a robust tool for evaluating trade-offs in stormwater infrastructure planning, advancing sustainable urban water management. G r in the context of older urban centers, the declining efficacy of aging infrastructure. These factors underscore the critical need for integrated and optimized design paradigms that not only ensure the reliability of GREI but also enhance its resilience and cost-efficiency, addressing the growing complexities of contemporary urban environments. The effectiveness of stormwater infrastructure is predicated upon the fulfillment of three core objectives: (a) reliable performance under standard hydrological conditions, (b) minimal failure frequency while adhering to predefined risk thresholds, and (c) robust resilience to extreme hydrometeorological events, such as severe storms and flooding [4]. In this context, the integration of green infrastructure (GI) emerges as a transformative approach, augmenting system resilience, flexibility, and adaptability while simultaneously mitigating environmental impacts [5]. By leveraging GI strategies, including bio-retention cells and green roofs, natural hydrological processes are mimicked, delivering a suite of co-benefits such as flood risk attenuation, the enhancement of water quality, groundwater recharge, and the mitigation of urban heat island effects [6]. However, it is imperative to acknowledge that GI is not a replacement for traditional GREI, but rather a complementary system, reinforcing the overall efficacy and sustainability of urban stormwater management frameworks [7; 8]. An optimal paradigm for urban stormwater management likely resides in the adoption of a Grey-Green Infrastructure (GGI) strategy, which synergistically integrates the sustainability and adaptability of GI with the engineering precision and reliability of GREI [9]. However, achieving an ideal equilibrium between GI and GREI represents a highly complex, multi-objective optimization challenge. This endeavor requires a holistic evaluation encompassing technological, economic, and social dimensions, while simultaneously addressing inherent trade-offs and conflicts among competing objectives. The intricacy of this optimization process underscores the need for advanced methodologies capable of navigating these multidimensional constraints to identify resilient and cost-effective solutions [10].