This article studies water wave diffraction and radiation by a submerged horizontal circular cylinder in front of a vertical wall under the assumption of linear potential flow theory. Based on the image principle, the hydrodynamic problem of a horizontal cylinder in front of a vertical wall is transformed into an equivalent problem involving symmetrical cylinders in a horizontally unbounded fluid domain. Then, analytical solutions for the present physical problem are developed using the method of multipole expansions combined with the shift of polar coordinate systems. The wave exciting forces on the cylinder as well as the added mass and radiation damping due to the cylinder oscillation are calculated. The analytical solutions converge very rapidly with the increasing truncated number of multipoles. Calculation examples are presented to examine the effects of different parameters on the hydrodynamic quantities of the cylinder. Results indicate that the hydrodynamic quantities of the cylinder in front of a vertical wall greatly differ from those in a horizontally unbounded fluid domain.