The design, analysis, and hydrotesting of a composite-aluminum cylinder joint subjected to external hydrostatic pressure is presented. This study addresses the feasibility of utilizing an adhesive bond to join composite and metallic pressure-hull components. A preliminary design methodology is outlined and applied to the joint. The design provides for axial load transfer through shear in the adhesive and bearing in the butt end of the cylinder. A thickened area of aluminum in the vicinity of the joint serves as a king frame to enhance stability of the structure. Detailed finite element analysis is performed to provide accurate failure predictions. Dimensional details of the joint are designed to obtain the desired ratio of shear/bearing load transfer and to minimize bending of the shell over the joint to delay interlaminar shear failure. An in situ thermoplastic filament-wound cylinder is manufactured and adhesively bonded to an aluminum cylinder. The instrumented structure is then tested to failure through application of external hydrostatic pressure. Experimental results including collapse pressure, failure mode, and strain field are compared with predictions to verify finite element model accuracy and validate the design methodology. The results of this study demonstrate both accurate predictive capability and use of an effective design methodology for composite pressure-hull joints.