The phase equilibria of the Al-Sc-Y system at 873 K have been determined by scanning electron microscopy/energy dispersive X-ray spectroscopy and X-ray diffraction using individual alloys. Phase transition temperatures for some of the annealed samples have been measured by differential scanning calorimetry. Before establishing the thermodynamic description of the ternary system, the phase diagram of the Al-Y system was re-optimized, showing an improvement in the calculated enthalpy of formation of the compounds compared with previous assessments. Based on the experimental results of this work and the re-assessed Al-Y binary phase diagram, the thermodynamic description of the Al-Sc-Y ternary system was developed combining the first-principles calculation and the CALPHAD method. With the obtained thermodynamic parameters, the isothermal section at 873 K, the vertical sections at 9 at.% Sc and 16 at.% Y, and the liquidus projection are all calculated, which account for most of the experimental information satisfactorily.