This research proposed a prismatic pressure vessel for gas-fuel storage on a gas-fuelled ship. The prismatic pressure vessel consisted of flat-plated shells, stiffeners, and beam structures. According to the design principle of pressure vessels, the beam structures were designed to receive tensile stresses generated from the internal pressure. International regulations and a finite element analysis were considered for the design, strength calculations, fatigue assessments, plastic collapse, and thermal effects. A case study in which the prismatic pressure vessel was applied as a gas-fuel tank to provide ship propulsion on a crude-oil tanker was performed. Nine per cent nickel steel was used to be applicable for cryogenic service. Several design loadings were considered to evaluate the design feasibility of the pressure vessel. The computational analysis results indicated that the prismatic pressure vessel satisfied the design regulations and should be applicable for use as a gas-fuel storage tank for ships.