This study investigates the influence of variable environmental conditions on the growth of the natural rock oyster (hooded oyster) Saccostrea cucullata in the river-fed Ambinanibe coastal lagoon (Madagascar), with the objective to establish sites with optimal growth conditions for aquaculture purposes. Specimen marked with manganese were placed in cages over an approximately one-year period. The cages were installed at four sites i.e. one at the river mouth, two sites within the main lagoon and one site in an adjacent connected lagoon, which is out of river influence and considered as the control site. Multiple mark and recapture experiments were used to investigate the variations in growth rate and condition index (tissue weight/shell weight). The results show that the species overall exhibits a fast growth rate at a young age, with a gradual growth reduction with age. The hinge growth rate shows important spatial and seasonal variations in the lagoon. Oysters from the site located at the river mouth have the highest hinge growth rate (20.5 ± 13.6 μm d− 1) and condition index (4.5 ± 2.8 %) while those from the control site out of river and ocean influences exhibit the lowest hinge growth rate (7.7 ± 4.2 μm d− 1) and condition index (0.7 ± 0.6 %), suggesting that river freshwater inflow provides suitable environmental conditions for oyster growth and a profitable oyster farming is thus feasible. River inflow is an important external source of nutrients, providing thus a significant food source (phytoplankton), as well as suitable temperature and salinity conditions for S. cucullata in this lagoon. Overall, our results show that oyster market size (shell length: 40 mm) in Madagascar can be reached faster at the river-influenced site (in 8 months) in comparison with the other sites (in 1 – 1.2 years), indicating that the river-impacted site is well suitable for oyster farming.