Chinese authority approved nirsevimab to prevent respiratory syncytial virus (RSV) in January 2024. We aimed to assess the cost-effectiveness of nirsevimab immunization among infants in Shanghai. A decision-tree Markov model was developed to compare two strategies - year-round and seasonal immunization - with nonintervention, from a societal perspective, using RSV-associated disease burden and demographic data. Twelve monthly birth cohorts were followed through 24 one-month Markov cycles. Two scenarios of nirsevimab efficacy were considered: short-duration and long-duration. The incremental cost-effectiveness ratio (ICER) (incremental costs per quality-adjusted life year [QALY] gained) was calculated and the willingness-to-pay (WTP) threshold was set at gross domestic product (GDP) per capita. Sensitivity analysis was performed to evaluate the uncertainty. Both immunization strategies demonstrated cost-effectiveness across efficacy scenarios, with seasonal approach yielding lower ICERs than the year-round approach. The cost-effectiveness of the seasonal approach was influenced by the timing of its administration. Nirsevimab immunization may be an economically favorable strategy for infant RSV prevention in Shanghai. The optimal program timing of seasonal immunization requires further investigation to maximize public health impact.