Emission from Ar III is seen in planetary nebulae, in H II regions, and from laboratory plasmas. The analysis of such spectra requires accurate electron impact excitation data. The aim of this work is to improve the electron impact excitation data available for Ar^2+^, for application in studies of planetary nebulae and laboratory plasma spectra. The effects of the new data on diagnostic line ratios are also studied. Electron-impact excitation collision strengths have been calculated using the R-Matrix Intermediate-Coupling Frame-Transformation method and the R-Matrix Breit-Pauli method. Excitation cross sections are calculated between all levels of the configurations 3s^2^3p^4^, 3s3p^5^, 3p^6^, 3p^5^3d, and 3s^2^3p^3^nl (3d<=nl<=5s). Maxwellian effective collision strengths are generated from the collision strength data.