The main aim of this paper was to report genetic parameters and selection response from a synthetic population of Nile tilapia selected for improved growth performance in brackish water systems in Vietnam. The synthetic base population was formed in 2007 from the best performing individuals for growth produced from a complete diallel cross involving three strains of Nile tilapia, namely GIFT (Genetically Improved Farmed Tilapia), Taiwan and NOVIT4 (GIFT-derived) strains. Selection was practised for increased harvest weight in brackish water (15 − 20 ppt) over four generations from 2008 to 2011. A total of 12,006 individuals had performance data records. They were offspring of 341 sires and 450 dams (averaging 3,000 offspring and 70 sires and 90 dams in each generation). Mixed models fitted to the data included the fixed effects of generation, sex, their two way interaction and a linear covariate of age within sex and generation. The random terms in the model were sire within generation and dam within sire and generation. The estimates of heritability for body traits and survival were moderate to high (0.27 to 0.53). Genetic correlations between harvest weight and body length were high and positive (0.97), whereas those between body traits and survival were low and not significantly different from zero. Genetic gain per generation was measured as estimated breeding values and expressed in actual units (original scale of measurement) and genetic standard deviation unit (σG). The improvement achieved for harvest weight ranged from 1.1 to 1.6 σG after four generations of selection (one year per generation). Selection for increased harvest weight was however accompanied by a non-significant decrease in survival by -0.24%-units or -0.16σG. The large genetic variation in both harvest weight and survival, however, suggest that there is scope for simultaneous improvement of both traits in this population of Nile tilapia. It is concluded that our selective breeding program has succeeded in developing a productive strain of Nile tilapia under brackish water systems, but the future work should include survival rate in the recording system, selection index and breeding objective.