We consider a scalar field theory in Minkowski spacetime and define a coarse grained Closed Time Path (CTP) effective action by integrating quantum fluctuations of wavelengths shorter than a critical value. We derive an exact CTP renormalization group equation and solve it using a derivative expansion approach. Explicit calculation is performed for the $λϕ^4$ theory. We discuss the relevance of the CTP average action in the study of non-equilibrium aspects of phase transitions in quantum field theory.

RevTeX file, 2 Postscript figures. Final version published in Phys. Rev. D - Equation (23) revised, and minor changes included