This paper investigates the acoustic boundary layer in porous-walled ducts with sidewall gas injection. The formulation, valid for low injection Mach numbers, is two-dimensional and includes the effects of viscosity, thermal conductivity, and steady-state velocities. Solutions are provided for the first-order axial and transverse velocities and temperature in the boundary layer in terms of the acoustic pressure. It is shown that with an increase in the steady wall injection velocity, the acoustic boundary-layer thickness at a given frequency increases and its tendency to damp acoustic motions within the duct also increases.