**Steven C. Frautschi** (; born December 6, 1933) is an American theoretical physicist, Professor of Physics at the California Institute of Technology. He is known for his contributions to the bootstrap theory of the strong interactions. He is a Fellow of the American Physical Society (2015).

In 1961 Frautschi, along with Geoffrey Chew, discovered that the mesons fall into straight-line Regge trajectories (in their scheme, spin is plotted against mass squared on a so-called Chew–Frautschi plot), and the two of them introduced the pomeron into the western literature. Frautschi's most well known contribution to strong-interaction theory was the **statistical bootstrap**, a prediction that the number of hadronic states grows exponentially with energy. This is nowadays understood as a manifestation of the deconfinement phase transition. The exponential growth is incorporated into string theory, where it is known as the Hagedorn temperature. (This S-matrix approach to the strong interactions was largely abandoned by the particle physics community in the 1970s in light of quantum chromodynamics.)

In 1961, with D. Yennie and H. Suura, he elucidated the role of infrared photons properly summed in high energy QED scattering.