Previous studies of the long-term climate effects of irrigation have focused on average monthly temperatures. Given the importance of temperature (T) extremes to agriculture and human health, we evaluated irrigation induced changes in various metrics of T extremes using daily observations in California and Nebraska. In addition, simulations from a regional climate model were used to evaluate irrigation effects on T and heat index (HI; also known as the discomfort index) extremes in California, with the latter representing a combined measure of T and humidity. Contrary to our expectation that irrigation would have larger effects on hot days when sensible heat fluxes are higher, both observations and a regional climate model indicate that irrigation cools T on the hottest days of the year by a similar magnitude as on an average summer day. The HI is also reduced by irrigation, but by a much smaller magnitude than T because of the higher humidity above irrigated surfaces. Interestingly, HI is influenced less on the most extreme days than on average days, because of the nonlinear effect of humidity on HI at high T.