A finite element method is presented for solving three-dimensional radiation problems in time-harmonic acoustics. This is done by introducing a so-called “Dirichlet-to-Neumann” boundary condition on the boundary of the domain discretized with finite elements. This DtN boundary condition is an exact non-reflecting boundary condition. It has been developed by Givoli and Keller [1, 2] for two and three dimensions. Calculations, however, have been carried out only for simple two-dimensional cases [2–6]. In this paper, the Dirichlet-to-Neumann boundary condition for problems in three dimensions is dealt with. From the strong form given by Givoli and Keller, the weak form is derived. Numerical examples show the applicability and performance of the DtN boundary condition.