The addition of carbon nanotubes (CNTs) to cement matrix brings multiple beneficial effects ranging from improving mechanical and physical properties to the creation of smart materials. When subjected to an erosive environment or as end-of-life waste, mortars with CNT addition might get released into the environment and come in contact with surface waters. The assessment of the environmental impact of mortars reinforced with carbon nanotubes is an important factor concerning their sustainability, as it has not yet been addressed in the literature. The presented paper aims to assess the water toxicity of cement mortars with various dosages of 0.05 wt.%, 0.1 wt.%, and 0.2 wt.% of carbon nanotube. The effect of the quality of water dispersion of CNTs was also considered through two sonication times of the suspension: 20 min and 60 min. Tests using indicator organisms,