In this presented work, pure and holmium, niobium, and holmium–niobium co-doped titanium dioxide nanopowders were synthesized by the sol–gel method. Their morphological, structural, optical, and photocatalytic properties were analyzed by scanning electron microscopy, X-ray diffraction, and UV–visible spectrophotometer. The photocatalytic activities of the synthesized nanopowders were examined using ultraviolet light. The results have shown that doping both with a single (holmium, niobium) or two types of elements (holmium–niobium) have significantly changed the properties of titanium dioxide. It was concluded that, compared with others, the holmium–niobium co-doped sample has the smallest crystal size. Other values, such as forbidden band gap and kinetic constants, are in the range of the data obtained for single-element doping.