We present a formulation for nonreflecting boundaries in fluctuating hydrodynamics. Nonreflecting boundary conditions are designed to evacuate sound waves out of the computational domain, thus allowing one to deal with open systems and to avoid finite size effects associated with periodic boundaries. Thermodynamic consistency for the fluctuation of the total mass and momentum of the open system is ensured by a fluctuation-dissipation balance which controls the amplitude of the sound waves generated by stress fluctuations near the boundary. We consider equilibrium and out-of-equilibrium situations (forced sound) in liquid water at ambient conditions and argon ranging from gas to liquid densities. Nonreflecting boundaries for fluctuating hydrodynamics make feasible simulations of ultrasound in microfluidic devices.