Single-crystalline orthorhombic antimony trioxide (Sb₂O₃) nanobelts with unique elliptical cross sections and purple-blue photoluminescence have been synthesized. The uniform Sb₂O₃ nanobelts are 400–600 nm in width, 20–40 nm in thickness at the center and gradually become thinner to form sharp edges sub-5 nm in size, tens of micrometers in length, and with [001] as the preferential growth direction. Self-assembly of tens of nanobelts into three-dimensional (3-D) flower-like nanostructures has been observed. Analysis was performed by X-ray diffraction, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy, selected area electron diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, and photoluminescence spectroscopy. The Sb₂O₃ nanobelts display intense purple-blue photoluminescence centred at 425 nm (~2.92 eV). The successful synthesis of nanobelts with elliptical cross sections may cast new light on the investigation of the property differences between nanobelts with rectangular cross sections and those with other cross section geometries. The Sb₂O₃ nanobelts can be used as effective purple-blue light emitters and may also be valuable for future nanodevice design.