Studies of stratospheric temperature variability typically include seasonal, quasi-biennial oscillation, solar, and volcanic effects, but the response to El Niño–Southern Oscillation (ENSO) is less well recognized. Modeling work suggests that ENSO may produce effects on surface climate at high latitudes by interaction with the polar stratosphere, yet until recently, past work has often failed to find a statistically significant ENSO response in polar stratospheric temperature observations. Using zonal mean temperatures from several improved radiosonde data sets beginning in 1958, we show a significant El Niño cooling signal in the tropical stratosphere and warming signal in the Arctic stratosphere in winter. In the tropical stratosphere the difference of more than 1 K between El Niño and La Niña temperatures is similar in magnitude to the tropospheric warming signal. The significant signal, derived from regression analysis, of more than 4 K in the winter Arctic stratosphere is generally largest in the lower stratosphere and extends into the upper troposphere. The signal, with a maximum in late winter, accounts for 14% to 25% of stratospheric temperature variability at 100 mbar in Arctic winter in radiosonde and reanalysis data. Satellite-derived temperatures show significant El Niño cooling in the tropical stratosphere in boreal winter, but the warming signal in the Arctic stratosphere is not statistically significant in that data set.