Quasi-aligned Nb2O5 nanobelt arrays 300–500 nm wide, 20–40 nm thick, and 5–10 μm long are fabricated directly on a Nb foil using a hydrothermal reaction in a KOH solution followed by protonation and calcination treatment. The morphology of the quasi-aligned nanobelts established during the hydrothermal growth of the KNb3O8 nanobelts is preserved after the ion exchange process which converts the products to H3ONb3O8, and Nb2O5 is produced eventually by calcination. The photocatalytic activity of the synthesized Nb2O5 nanobelts is evaluated for the first time. The Nb2O5 nanobelts are further functionalized using Ag nanoparticles to form heterostructured Ag–Nb2O5 nanobelts by immersion in a AgNO3 solution followed by UV (ultra-violet) light illumination. The heterostructured Ag–Nb2O5 nanobelts consisting of Nb2O5 nanobelts and Ag nanoparticles exhibit enhanced photocatalytic activity and can decompose dye pollutants more rapidly and efficiently because Ag nanoparticles on the Nb2O5 surface can serve as electron sinks which promote charge separation between the photogenerated electrons (e−) and holes (h+). In addition, the composite photocatalyst exhibits excellent antibacterial activities against both Escherichia coli (gram-negative) and Staphylococcus aureus (gram-positive) and is potentially useful in photodegradation of organic dye pollutants and antibacterial applications.