This paper uses nonlinear ultrasonic measurements to monitor creep-induced microstructural changes in concrete. A new NDE approach that enables the in-situ monitoring of the damage state in concrete is developed in which the second harmonic generation (SHG) technique using nonlinear Rayleigh surface waves is adapted to a cylindrical specimen. This cylindrical specimen is under a uniaxial compressive load (70% of the ultimate strength). The acoustic nonlinearity parameter, β is measured as a function of creep time. The following conclusions are drawn from the experiments: (1) the results suggest that the proposed NLU technique based on the SHG theory (β) is feasible in concrete and this approach shows the expected trends in the behavior of the fundamental and second harmonic amplitudes with respect to propagation distance; (2) the measured nonlinearity parameter, β is highly sensitive to creep- induced changes in the microstructure; and (3) unlike conventional strain based creep monitoring methods, the nonlinearity parameter, β gives a clear indication of the secondary stage of creep. Consequently, it is demonstrated that the time-dependent creep damage in concrete can be monitored with the proposed SHG method. These results can be used to study the microstructure behavior of concrete under creep through a mechanistic model and illustrate the potential of SHG for the in situ monitoring of creep in concrete structures.