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Iron-based nanoparticles (FeNPs) have been used successfully in water treatment and environmental cleanup efforts. This study examined ecotoxicity of two FeNPs produced with extract from green tea (smGT, GTFe) and their ability to degrade malachite green (MG). Their physicochemical properties were assessed by transmission electron microscopy, X-ray powder diffraction, dynamic light scattering, and transmission Mossbauer spectroscopy. Using a battery of ecotoxicological bioassays, we determined the toxicity for nine different organisms, including bacteria, cyanobacteria, algae, plants, and crustaceans. Iron and iron oxide nanoparticles synthesized with green tea extract displayed low capacity to degrade MG and were toxic to all tested organisms. Superparamagnetic iron oxide nanoparticles (smGT) derived from GTFe showed no toxic effect on most of the tested organisms up to a concentration of 1 g/L, except for algae and cyanobacteria, and removed 93% MG at a concentration of 125 mg of Fe/L after 60 min. The procedure described in this paper generates new nontoxic superparamagnetic iron oxide NPs from existing and toxic GTFe that are endowed with degradative potential for organic compounds. These findings suggest low ecotoxicological risks and the suitability of these green-synthesized FeNPs for environmental remediation purposes.