A relativistic fluid theory is constructed to analyze the inverse Compton scattering of an unmagnetized counterpropagating plasma wave by a cold relativistic electron beam. A Langmuir wave and electromagnetic plasma wave wigglers are considered, and the properties of the two types of wigglers are comparatively discussed. The growth rates are comparable to those of the ac free‐electron laser and the free‐electron lasers with magnetostatic wigglers in the Raman regime.