To elude the toxic effects of chemically synthesized nanoparticles, the phytochemically synthesized nanoparticles may provide a better alternative. For the first time, an aqueous extract of Juncus inflexus shoot with FeCl3.6H2O was used for the phytosynthesis of iron oxide nanoparticles (FeONPs). As-synthesized FeONPs were characterized by UV-Vis spectroscopy, Transmission electron microscopy (TEM), Dynamic light scattering (DLS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). FeONPs showed UV-vis absorption spectra between 300-400 nm, whereas TEM analysis confirmed the particle sizes of 40-60 nm with aggregation. XRD is confirming the polymorphic composition of Fe3O4, α-Fe2O3, and Fe0 nanoparticles. Furthermore, FTIR analysis presenting the most probable mechanism for the synthesis of FeONPs. This multiphase FeONPs was applied for the decolorization of methylene blue dye (>83%). Phytosynthesized FeONPs have the benefits of low cost, no toxicity, sustainable, and eco-friendly technology so that they may be used as adsorbent/catalyst for remediation of toxic dyes in an aqueous medium.