A technique based on the discontinuous Galerkin finite element method is developed and applied to the derivation of an absorbing boundary condition for the analysis of transient wave propagation. The condition is exact in that only discretization error is involved. Furthermore, the computational cost associated with use of the condition is an order of magnitude lower than for conditions based on Green functions. The time-stepping scheme resulting from an implicit method in conjunction with this boundary condition appears to be unconditionally stable.