Contamination of water resources due to the toxic industrial waste materials is increasing due to the unsustainable water treatment processes. Simple photocatalytic water treatment methods often cause secondary pollution due to the residual catalysts. These residual catalysts are mostly nanosized and absorbed by the aquatic life posing risks to human health and ecosystems. In this article, heterogeneous nanoparticles were successfully synthesized by capping Fe3O4 nanoparticles with CdSe using simple co-precipitation method at temperatures less than 80 °C. These nanoparticles can be used as a photocatalyst to remove organic pollutants from waste water upon the irradiation of natural sunlight, and can be retrieved using external magnet from the treated effluent. The heterostructure of the CdSe–Fe3O4 nanoparticle is confirmed using the HRTEM analysis and the average size is found to be less than 10 nm. The photocatalysis of methylene blue dye using this synthesized magnetic nanoparticle shows a color removal efficiency of 98.33 % within 150 min of exposure to natural sunlight. Also, this photocatalyst exhibits a super paramagnetic behavior with a magnetic strength of about 11.6 emu/g and a color removal efficiency of 84.49 % is been retained after 6 cycles of reusability study.