An absolute flux calibrated reference spectrum of the Sun covering the 0.12-2.5 μm wavelength range is presented. The ultraviolet and optical spectrum is based on absolute flux measurements from satellites and from the ground. The near-infrared spectrum is based on measurements using the NASA CV-990 aircraft and on a model spectrum. The synthetic optical and near-infrared magnitudes of the absolute calibrated solar reference spectrum agree with published values to 0.01-0.03 mag, i.e., within the uncertainties of the measurements. The absolute flux of the reference spectrum over the optical and near-infrared 0.4-2.5 μm range is known with an uncertainty of 5%, or better. In the blue and ultraviolet, especially for wavelengths in the 0.12-0.2 μm interval, the uncertainty increases up to about 20% due to the variability of the solar energy output at these wavelengths. The absolute flux spectrum of the Sun presented here will help to establish the absolute calibration of NICMOS, the HST near-infrared camera and multi-object spectrograph.