Despite increased interest in the development of safe and greener nanomaterials for use in different technologies and applications, there are still gaps in our knowledge regarding the potential impacts of new nanoformulations on aquatic organisms and the environment. To address some of these gaps, this study evaluated the ecotoxic effects of agriculturally relevant iron nanoparticles on zebrafish (Danio rerio) embryos. Two kinds of iron oxide nanoparticles, biogenic and chemically synthesized, were investigated for their potential acute and sub-lethal effects on zebrafish embryos at increasing test concentrations. Dose-dependent experiments revealed no adverse effect on zebrafish embryo survival at concentrations ranging from 25 µg/mL to 500 µg/mL. Similar to the effect on embryo survival, the test nanoparticles were observed to have no effect on embryo hatching. A concentration-dependent decrease in heart rate was observed for the nanoparticles tested, however, the zebrafish embryos heart rate was still in the normal physiological range. Observations made for developmental deformities including yolk sac edema, pericardial edema, and spinal curvature in zebrafish embryos exposed to nanoparticles confirmed no toxic effects for all tested nanoparticles. Analysis for sub-lethal toxic effects caused by iron nanoparticles using real-time polymerase chain reaction technique (RT-PCR) showed no statistically significant up-regulation of superoxide dismutase (SOD) and catalase (CAT) genes involved in oxidative stress. Alkaline phosphatase staining of embryos after exposure to the nanoparticles enables blood vessels to be visualized using microscopy to assess any sub-lethal effects on blood vessel development. Overall, the study demonstrated that both iron oxide nanoparticles tested caused no acute toxicity and minimal sub-lethal toxicity in zebrafish embryos.