<p>A waveguide port boundary condition (WPBC) based on the restriction of the approximation space is presented in the context of Finite Element Analysis. As well as efficiently truncating the computational domain in the same manner as the traditional WPBC, the proposed scheme significantly reduces the degrees of freedom at the waveguide ports, simplifies the implementation and seamlessly provides post-processing results such as the reflected power in each waveguide mode. The boundary condition is thoroughly derived, and numerical examples are used as a support for the discussion on topics such as the needed number of modes to be employed at a waveguide port. Finally, a nanograting-based plasmonic sensor is analysed to illustrate further possibilities of the scheme.</p>