Arabian Gulf seawater is characterized by its high average seasonal temperatures, silt density index (SDI), and total dissolved solids (TDS), in addition to other parameters such as level of pollution and type and magnitude of marine microorganisms. The overall effect of these on RO membrane performance is very significant and complex as well. Hence, the primary objective of this paper is to present a mathematical model for the prediction of RO membrane performance for Arabian Gulf seawater under the prevailing local conditions. A complete system of equations that can be used for the prediction of product water now rate and separation of ions present in seawater for a given RO membrane was obtained on the basis of the Kimura-Sourirajan analysis. Solution of this system for the purpose of performance prediction requires prior knowledge of certain parameters that are specific to the RO membrane under consideration and pertinent to the ions present in the seawater. A computer program has been developed to evaluate such parameters, solve the obtained system of equations, and process the performance predictions utilizing experimental data available from the operation of two RO membrane systems, spiral wound and hollow fiber, at the Doha Desalination Research Plant (DRP). Comparisons between predicted and experimental data at standard conditions have shown reasonable agreement for product flow rates and very good agreement for the rejection of total salts and ions present in the seawater.