Abstarct: In gas reservoirs, the produced water usually accompanies gas production. Most of the reservoirs at the beginning of production have fresh water; but with passing time due to the increase in productions and decrease in pressure of reservoir, the produced water gradually becomes saline. The saline produced water causes severe corrosion in pipelines and well head facilities leading to reduction in gas productions. Determining the origin of salinity in oil and gas reservoirs is most important for controlling and managing this problem and consequently increasing the gas production. Shanoul gas reservoir is one of the Parsian gas fields in southern Iran in which produced saline water via some gas wells, is causing reducing the gas productions. This reservoir is composed of Kangan and Dalan formations. The present research aims to determine the brine origin with the data of isotopic and hydrochemical analysis. The potential processes causing salinity can be halite dissolution, membrane filtration and evaporated sea water. The potential sources of water may be meteoric, present or paleo-sea water. The mixing between ancient evaporated sea water and condensed vapors is determined as the main origin of salinity in the Shanoul gas reservoir using hydrogeochemistry, isotope and geological methods. There are two types of waters in the reservoir that have ancient evaporated sea water origin; the brine under the reservoir (aquifer water) and intergranular water which fills the pore spaces in gas reservoir. baxsed on isotopic analysis intergranular water is the main source of salinity that has entered in pay-zone as the result of passing time and decrease in pressure of reservoir; so there isn’t any up-coning of aquifer saline water. The chemical evolution of the saline produced waters is affected by three different processes: evaporation of seawater, water–rock and water–gas interactions. Due to the evaporation process, the concentration of all ions in the produced waters increased up to saturation levels. In comparison to the evaporated seawater, the higher concentrations of Ca, Li, Mn, I and Sr and lower concentrations of Mg, K, SO4 and Na ions are observed in the Shanoul produced waters.