The purpose of the Musaimeer Pump Station and Outfall (MPSO) project is to receive storm water runoff and groundwater flows from the Abu Hamour Tunnel and to provide a pumping station, a marine outfall tunnel and a diffuser field to dispose of the flows into the sea. Musaimeer outfall tunnel is one of the longest storm water tunnels in the world with a total length of 10.2 km. The tunnel is connected via a drop shaft to the main pump station. This system will accommodate surface and storm water received from the drainage networks of 190 km2 of urban areas in southern Doha. The outfall tunnel comprised precast segmental lining with internal diameter of 3700 mm sloped at 0.05% upward to the riser shaft. The riser shaft, which is located at the end of the outfall tunnel, is connected to a diffuser field positioned on the Arabian Gulf seabed. This paper will focus on the design considerations and recommendations related Marine outfall riser shaft, distribution manifolds and diffuser field. to The MPSO marine outfall was designed in two phases, first comprising installation of riser shaft, primary and secondary manifolds, 3 lines of HDPE piping with variable internal diameter, 84 diffuser duckbill valves, 84 diffuser protection boxes and scour protection. The diffuser is laid in an area of 280mX40m 15m beneath the sea water level. Second phase of marine outfall design included the meticulous connection works between GRP riser shaft and 10.2km TBM tunnel 15m beneath the seabed. Design considerations and final conclusions of such marine design works will be presented.