Characterization of long-term solar ultraviolet (UV) irradiance variations is important for understanding the radiative forcing of Earth's atmosphere. Extending such a characterization to multidecadal timescales requires the merging of multiple satellite data sets. Currently available irradiance data sets show both absolute offsets and time-dependent differences that vary between spectral ranges. We have created a composite spectral UV irradiance data set with daily spectra covering the wavelength range 120–400 nm for the time period November 1978 to August 2005. A reference irradiance spectrum is used for normalization to remove absolute biases between instruments. We selected the best irradiance data set for each spectral and temporal interval based on intercomparisons between overlapping data sets and comparisons with proxy model predictions. The long-term behavior of the merged composite irradiance data set is consistent with expectations based on other solar activity indicators. There is no difference between the minimum values for solar cycles 21 and 22 within the uncertainty of the data, and the maximum irradiance values for cycle 23 are slightly lower than those observed for cycles 21 and 22. We also compare our composite data set with published synthetic irradiance data. These two data sets generally agree within ±2% over most spectral and temporal ranges. We hope to extend this data set in the future using measurements from the SORCE satellite.