A new visible-light-driven photocatalyst, monoclinic dibismuth tetraoxide (m-Bi2O4), has been discovered. This simple oxide with mixed valent states (Bi3+ and Bi5+) has a band gap energy of 2.0 eV, and is responsive to wavelength up to 620 nm. The first-principle calculations within the hybrid density functional framework (HSE) indicate that m-Bi2O4 is a typical indirect-gap semiconductor. It shows much higher photocatalytic degradation activity as well as photostability than some of the well-investigated highly efficient photocatalysts, such as CdS and Bi2O3. Not only typical dyes and colorless aromatic compounds, including rhodamine B, methyl orange, methyl blue, phenol and 4-nitrophenol, but also bacterial (Escherichia coli) cells can be effectively degraded/inactivated in the presence of m-Bi2O4. The electronic potential of band structure is studied and a photocatalytic mechanism is also proposed, in which both (OH)-O-center dot and direct h(+) oxidation play important roles in the degradation process. Our discovery highlights the potential of developing Bi2O4-based photocatalysts as a new class of highly active, stable and safe photocatalyst for versatile pollutant degradation and disinfection under visible light. (C) 2015 Published by Elsevier B.V.