We report for the first time the synthesis of colloidal Ln³⁺/Yb³⁺ co-doped KY₃F₁₀nanocrystals (Ln = Er or Eu) viathermal decomposition. The transmission electron microscopy (TEM) studies indicated that the average size of the nanocrystals was approximately 15 nm. The nanocrystals crystallized in the cubic Fmm space group as confirmed by Rietveld refinement of the X-ray powder diffraction patterns and by electron diffraction analysis. The Eu³⁺ ion was used as a spectroscopic probe and its emission spectrum in colloidal KY₃F₁₀nanocrystals showed that the lanthanide ions replaces Y³⁺ in the lattice structure. When co-doped with Er³⁺/Yb³⁺, colloidal KY₃F₁₀nanocrystals exhibited visible (green and red) upconversion emissions following 978 nm excitation with a near-infrared (NIR) diode laser. Power dependence studies indicated that a two-photon energy transfer upconversion process was responsible for the NIR-to-visible upconversion.