Two consecutive trials were conducted to evaluate the effects of guanidinoacetic acid (GAA) supplementation (a creatine precursor) and energy levels in broiler diets based on maize-soybean meal (Trial 1) or that additionally included poultry by-product meal (PBPM; Trial 2) on growth performance, carcass yield and breast meat quality to 41 days of age.A total of 792, one-day old male Ross 308 broiler chickens were randomly distributed into six treatments - three energy levels (sufficient AME or 0.2 and 0.4 MJ/kg reduced AME) and two GAA levels (0.00 or 0.06%) with eight replicates for each trial.Reducing dietary energy resulted in poorer body weight gain (BWG) and feed conversion ratios (FCR) for each trial (P<0.05). However, GAA supplementation improved FCR, BWG and European Production Efficiency Factor (EPEF) (P<0.05).Dietary energy level and GAA addition had no significant effect on carcass parameters, drip loss, pH and chemical composition of breast meat (P>0.05), but decreased relative liver weight (P<0.05).It was concluded that, regardless of dietary energy levels, supplementation of GAA to plant-based diets or those including PBPM has the potential to improve growth performance in broilers.