In this paper, we present a nonoverlapping optimized Schwarz algorithm with a highorder transmission condition for the Helmholtz equation posed in waveguides. We introduce the new high-order transmission conditions based on the complete radiation boundary conditions (CRBCs) that have been developed for high-order absorbing boundary conditions. We obtain the convergence rate of the algorithm in terms of reflection coefficients of CRBCs, which decrease exponentially with the order of CRBCs. It will be shown that damping parameters involved in the transmission conditions can be selected in an optimal way for enhancing the convergence of the Schwarz algorithm. Also, this algorithm can be employed efficiently for a preconditioner in GMRES implementations based on the substructured form as in [M.1. Introduction. In this paper, we introduce a new high-order transmission condition for the nonoverlapping Schwarz method for the Helmholtz equation posed in waveguides. The Schwarz method [22, 24] is a well-known numerical technique that can provide an efficient solver for treating large scale problems arising from physics and engineering by allowing parallel computations. In developing an optimized Schwarz algorithm without overlap for the Helmholtz equation, it is important to design a highorder transmission condition for communication between neighboring subregions. To do this, we introduce a new transmission condition based on the complete radiation boundary conditions (CRBCs) designed for high-order absorbing boundary conditions for time-harmonic wave propagation problems in [15,16].We consider a nonoverlapping Schwarz algorithm in a waveguide in R d with d = 2 or 3. The model problem is to find the solution u satisfying the Helmholtz equation in a waveguide Ω = {(x, y) ∈ R d : x ∈ R and y ∈ Θ} with Θ a bounded domain in R d−1 with Lipschitz boundary ∂Θ: