In this study, titanium-clad bimetallic steel (TCBS) was innovatively introduced into the plate girder as a web, which enhanced the durability. The ultimate strength of a longitudinally stiffened TCBS plate girder under patch loading was investigated. There were clear differences in the nonlinear properties of the stress–strain curves of Q235 carbon steel, TA1 titanium, and TCBS. The numerical modeling method was verified via an experiment on a TCBS plate girder. A parametric study was performed on the ultimate strength of a longitudinally stiffened TCBS plate girder. The key parameters selected in this study included the position of the longitudinal stiffener, web aspect ratio, web slenderness, and relative load length. The variations in the ultimate strength and the buckling reduction coefficient were examined. The results indicated that the effects of the key parameters on the ultimate strength were clearly different from those on the buckling reduction coefficient. Additionally, the design equations in EN 1993–1–5 were too conservative to be suitable to predict the ultimate strength of the TCBS plate girders subjected to patch loading. Based on the numerical results, a predictive equation for the buckling reduction coefficient of a longitudinally stiffened TCBS plate girder under patch loading was devised. Compared with design equations in EN 1993–1–5, the proposed equation showed better accuracy.