Zinc oxide nanoparticles were synthesized using three techniques of combustion, hydrothermal and precipitation in the presence of Cystoseira brown algae extract and in the absence of it. The XRD results showed that the synthesized samples have a hexagonal wurtzite crystal structure, and the nanoparticles synthesized in the presence of algae, in all three techniques, have smaller crystal sizes. The FE-SEM images showed that the nanoparticles synthesized by the combustion method did not have specific geometric shapes and the accumulation of particles, synthesized in the absence of algae extract was higher. FE-SEM image of nanoparticles synthesized by hydrothermal method shows two different shapes. Synthesized particles in the presence of algae extract are observed as porous plates and those prepared in the absence of it are observed as cylinders with hexagonal cross-sections. The nanoparticles obtained by precipitation method in the presence of algae extract, accumulate in a hierarchical form, while those obtained in the absence of it were spheres with various dimensions. The specific surface area, obtained by BET, for nanoparticles synthesized by hydrothermal technique in the presence of algae extract, was more than other samples. Nitrogen gas adsorption and desorption diagrams for all synthesized samples are of type IV with hysteresis loop of type H3, corresponding to a mesoporous texture. The band gap, calculated based on the UV-Vis spectra, is smaller for the samples synthesized in the presence of algae extract. Electrochemical behavior of zinc oxide nanoparticles was investigated by electrochemical impedance spectroscopy and cyclic voltammetry, and the lowest resistance and the highest electroactivity were observed for the nanoparticles synthesized by hydrothermal technique in the absence of algae extract. The photocatalytic activity of zinc oxide nanoparticles for the removal of methylene blue, indigo carmine and bromocresol dyes under sunlight, ultraviolet and