Nanotechnology progress is beyond all borders and undoubtedly has improved the well-being of humanity. Among the novel high-tech materials, nanoparticles (NP) are one of the most important due to their wide range of applications. However, their synthesis methods often implicate toxic reagents that ultimately reach the environment. Therefore, other more “ecofriendly” alternatives have been proposed, leading to the “green-synthesized” NP (GS-NP). In spite of GS-NP having recently gained importance with remarkable results in terms of antioxidant properties and efficacy, the concern about the lack of toxicological studies has generated a debate regarding their actual ecosafety. This chapter aims to provide an integrative overview of GS-NP in terms of (i) their physical and chemical characteristics and applications, (ii) describing the main toxicity mechanisms, (iii) evaluating the up-to-date scientific production with a focus on their ecotoxicity in aquatic organisms through a bibliometric analysis, and (iv) identifying the gaps in knowledge of GS-NP toxicity from an environmental perspective. The vast majority of GS-NP analyzed in terms of ecotoxicity were plant- and metal-based, the first and second places being Ag- and Zn-based NP, respectively. The most studied group are the fish, followed by the microcrustacean. However, marine and/or estuarine species are poorly represented. A wide variety of ecotoxicological endpoints proved the deleterious effects of GS-NP on aquatic organisms such as oxidative stress, genotoxicity (and altered gene expressions), tissular damage and histopathology, tissue accumulation, and hatching and survival alterations in fish larvae. For microcrustacean, mortality was well assessed, together with reproductive impairments and bioaccumulation. Overall, we highlight the need for further ecotoxicological assessments in terms of nano-ecotoxicity of these green chemical-based materials since, despite having been suggested to be ecofriendly, their deleterious effects on aquatic biota were proved and could be underestimated.