In recent years, subsea tunnels have been constructed at greater depths, higher groundwater tables, and longer drives than those built previously. Controlling the water pressure and inflow rate of groundwater, i.e., the design of the drainage system, is a major challenge in subsea tunnels in terms of achieving hydraulic stability and ensuring economical operations. This has significant cost implications during construction and operation for long and deep subsea tunnels. In this study, the drainage system of subsea tunnels is optimized using the particle swarm optimization (PSO) algorithm, in which hydraulic boundary conditions, including size of the tunnel, number of sump pits and pumps, and pump operation plans, are allocated section-by-section and simulated to achieve economic optimization. Additionally, parametric studies are conducted to evaluate the influencing factors, including the drainage condition, hydrostatic pressure, radius of the tunnel, and initial stress. It is shown that optimal drainage is established under full drainage, nondrainage, and sometimes partial drainage systems depending on the hydraulic conditions. A tentative application of the proposed simulation method to a specific subsea tunnel illustrated that the construction cost can be significantly reduced through the optimization of the tunnel drainage system. [ABSTRACT FROM AUTHOR]