No, decaffeinated coffee is not a diuretic. Decaf coffee should have 97% of its caffeine content removed therefore it should not be considered a diuretic. It appears that some decaf coffee offered in coffee shops may still have some caffeine content as it was shown in this study by the University of Florida. Therefore, it may still have caffeine traces and if you are very sensitive to caffeine and you may feel some effects. It all depends on the efficiency of the decaffeination process.