In this paper, a parallel-plate ladder waveguide (PPLWG) is proposed for terahertz waveguiding. A modal analysis of the proposed structure is performed by the use of the generalized multipole technique and is verified using both the mode matching technique and the finite-element method. It is shown that guided modes of this waveguide are ${\text{TE}}_y^m$ even modes. The dispersion diagram, mode field distribution, and attenuation constant of the dominant mode are also presented. It is shown that by a proper choice of PPLWG dimensions, its mode confinement and dispersion profile can be adapted to typical requirements. Moreover, the field distribution of the dominant mode of a PPLWG can be matched to the ${\text{TE}}_1$ mode of a parallel-plate waveguide which is of practical importance.