A facile sol–gel technique was employed to synthesize the nanoparticles of β-Bi2Mo2O9 in acidic and water medium and Eu doped β-Bi2Mo2O9 in acidic medium. The crystalline structure of the synthesized nanoparticles was characterized by powder X-ray Diffractometry, thereby confirming the monoclinic structure of pure β-Bi2Mo2O9 and Eu-doped β-Bi2Mo2O9. Raman analysis was performed to verify the phase purity and to identify the vibrational modes of synthesized pure and doped molybdates. The surface morphology of synthesized particles was examined using scanning electron microscopy, which shows the formation of a honeycomb-like structure with numerous pores. UV–visible spectroscopy and XPS studied the optical properties of synthesized nanoparticles. The dye degradation process demonstrated the photocatalytic activity under visible light radiation in which Methylene blue acts as a model dye. The change in the absorbance of the dye with synthesized β-Bi2Mo2O9 and Eu-doped β-Bi2Mo2O9 with different concentrations of Eu3+ was studied by a UV–Visible spectrophotometer. The results indicate that the water-based β-Bi2Mo2O9 shows more dye degradation than acidic-based β-Bi2Mo2O9. The doping of Eu3+ increased the photocatalytic activity of acidic-based β-Bi2Mo2O9 synthesis, and the efficiency increased with an increase in the doping percentage of Eu3+. The 10% Eu:Bi2Mo2O9 completely degraded the Methylene Blue dye (100 %) within a period of 4 h.