Researchers use ultrasound (US) to modulate diffusive light in a highly scattering medium like tissue. This paper analyzes the US–optical interaction in the scattering medium and derives an expression for the US-modulated optical radiance. The diffusion approximation to the radiative transport equation is employed to develop a Green’s function for US-modulated light. The predicted modulated fluence and flux are verified using finite-difference time-domain simulations. The Green’s function is then utilized to illustrate the modulated reflectance as the US–optical interaction increases in depth. The intent of this paper is to focus on high US frequencies necessary for high-resolution imaging because they are of interest for applications such as phase conjugation.