A concrete-steel mixed structure combines steel framing elements with reinforced concrete in its framework to carry the structure's weight and to resist lateral forces (like wind and seismic shocks). This can take various forms, including a concrete core surrounded by steel beams, steel outrigger beams used with a concrete structure, or steel beams encased in concrete.

Concrete-steel mixed framing systems are most common in large structures such as bridges and skyscrapers. Its use in tall buildings has increased since the 1960s. In a mixed structure, steel and concrete work together to balance their advantages, so that neither material predominates over the other.

The presence of steel reinforcing bars in concrete does not make a structure mixed material; this is simply reinforced concrete. Also, concrete floorplates in a steel structure do not normally act as framing elements, and do not determine the frame material.