• MnO 2 nanoparticles have been synthesized using a facile and sustainable strategy. • An edible freshwater red alga, Lemanea manipurensis has been used. • The MnO 2 NPs showed enhanced photocatalytic activity. Composition controllable, facile and ambient temperature green synthesis of high purity manganese dioxide (α-MnO 2 ) nanoparticles has been accomplished via reduction of an aqueous KMnO 4 solution using aqueous extract of an edible freshwater red alga, Lemanea manipurensis . The UV–vis spectrum exhibited a broad band centered at 368 nm and the band gap value obtained from the Tauc plot is 2.44 eV. The TEM analysis confirmed the predominant spherical morphology with extensive nanotwinning of the nanoparticles in the size range 6–28 nm. The FE-SEM depicted as spherical agglomerated morphology. The average hydrodynamic diameter of the nanoparticles is found to be ∼115 nm from DLS study. The X-ray diffraction (XRD) spectrum showed sharp intense peaks which correspond to the crystallization of phase-pure α-MnO 2 nanoparticles in tetragonal crystal morphology. The infrared spectroscopy provided clear evidence of the presence of phytomolecules which are responsible for the stabilization of the nanoparticles. The as-synthesized MnO 2 NPs showed excellent photocatalytic activity for degradation of toxic dyes, rhodamine B (RhB), methyl orange (MO) and methylene blue (MB), the degradation rate being 0.06781, 0.04323 and 0.03831 min −1 , respectively. The photocatalyst is readily recoverable and showed excellent stability even after four cycles. The best photocatalytic performance was observed for RhB degradation under scattered sunlight and with 3 mg MnO 2 NPs exhibiting almost complete degradation (92%) within 30 min of exposure time. The photocatalytic degradation for all the dyes followed a pseudo-first order kinetics.