The performance of steel structures strengthened with externally bonded fiber-reinforced polymer (FRP) rely heavily on the interfacial shear stress transfer mechanism of the FRP-to-steel bonded interface. Much is known about the behavior of FRP-to-steel bonded joints under mechanical loading, but little is known about the performance of this type of bonded joints at elevated temperatures. Almost all adhesives typically used in FRP-to-steel applications experience a change in their mechanical behavior at temperatures <70°C. Therefore, gaining a sound understanding of the behavior of FRP-to-steel bonded joints at elevated temperatures is necessary. This paper presents a series of tests where carbon FRP (CFRP)-to-steel bonded joints are subjected to elevated temperatures. The outcomes of this paper showed that, at elevated temperatures, the dominant failure mode of the CFRP-to-steel bonded joints is the cohesion failure within the adhesive. The bond strength was found to increase with the temperature until the heat deflection temperature (HDT). The bond–slip behavior of the interface was found to undergo significant changes with increasing temperature. Specifically, the initial elastic stiffness and the peak shear stress were found to decrease as the temperature increases. The fracture energy was found to increase at temperatures below the HDT but then decrease drastically when the temperatures exceed the HDT.