Mariculture provides an increasing seafood supply to a growing population. It also brings unintended consequences for the environment, resources and sustainable development. Based on observation of environmental changes in intensive mariculture of lobster cages in three south-central regions of Vietnam (namely Xuan Dai Bay, Van Phong Bay and Cam Ranh Bay, each region had two farming stations and one reference station), this paper aims to determine the impacts of mariculture to environmental quality over-period of April 2019 to May 2020. The results indicated no differences in water quality within the bay and among bays (p>0.05), but significant differences in sediment quality among stations (with and without mariculture) within the bays and among bays. The accumulation of observed parameters in substrate sediment at mariculture regions was higher than that without mariculture (p<0.05). Within the Cam Ranh Bay, the concentration of organic carbon, total nitrogen, and total phosphorus of the sediment near the cages increased by a factor of 1.4 compared with the reference. Similar results were also observed in the case of Van Phong Bay and Xuan Dai Bay with different magnitudes. Additionally, based on changes in sulfide concentration of substrate sediment, the sediment quality in Van Phong Bay would be better than two other bays This evidence demonstrated that mariculture wastes accumulated in the substrate sediment, and over time they decomposed and created sub-toxins that degraded substrate and affected benthic biota in coastal waters.