While the trend towards building resilience in transport infrastructure systems in the face of extreme flooding is accelerating, scant attention has been paid to assessing the flood resilience of urban rail transit systems (URTSs). This paper develops an approach to quantitatively assess the URTS flood resilience by integrating the system performance-based resilience triangle curve, complex network modelling, and scenario analysis. A case study of the London URTS has been demonstrated to test its feasibility. This approach is novel in capturing the structural topology, operational performance, and engineering features of the URTS to comprehensively assess its resilience under flooding.