The wastewater treatment problem of Marmaris District is aimed to be solved through planning of an additional treatment plant with a capacity of 59.656 m3 /day by calculation of the population increase in 2047. The existing Wastewater Treatment Plant is designed with activated sludge process with a capacity of 50,625 m3 / day. Treated waste waters are discharged through sea outfall with a depth of 20 m in Yalancı Boğaz Region. Due to Touristic reasons, summer and winter periods were taken into consideration and two different working scenarios are planned during design period.