The planned Jeju undersea railway tunnel project aims to connect Jeju island and the Korean peninsula with intermediate stops at the islands of Bogildo and Chujado. The proposed 167 km TBM tunnel route passes through a wide spectrum of geological conditions ranging from intact rock to uncondolidated soils. The excavation of undersea pilot tunnels in highly jointed rocks and unconsolidated soils entails rigorous pregrouting during the TBM drive to reduce high water inflow. Weak zones in the planned tunnel route are also grouted in advance before the excavation of the main tunnels to ensure safe construction conditions. Grouting arrays at the tunnel face during the drive have often been used in TBM excavated tunnels under high water heads. Determination of successful grouting relies on Lugeon tests or coring. However, these techniques disrupt the surrounding environment and can be difficult to achieve under high water heads. In addition, cementitious grouts are susceptible to leaching and weathering under subsea conditions due to the high salt content and underwater currents. This can result in long term stability problems for undersea tunnels. Maintenance monitoring of the grouted zones along the tunnel route is essential in determining the long term stability of undersea tunnels. Advancements in geophysical non-destructive testing techniques have led to grout monitoring using elastic wave velocity and electrical resistivity. Both techniques have been extensively verified at inland drill-and-blast sites to determine the success of grouting. This study introduces the conceptual design of a monitoring sensor for grout zones in TBM excavated undersea tunnels. The sensor consists of a pipe-shaped casing with exterior sensor attachment that can measure the elastic wave velocity, electrical resistivity and water pressure of the surrounding grout zone. The elastic wave velocity and electrical resistivity measurements are used to determine the stiffness and hydraulic characteristics of the grout zone.