In theoretical physics, type I string theory is one of five consistent supersymmetric string theories in ten dimensions. It is the only one whose strings are unoriented (both orientations of a string are equivalent) and which contains not only closed strings, but also open strings.

It can be obtained as an orientifold of type IIB string theory, with 32 half-D9-branes added in the vacuum to cancel various anomalies.

At low energies, type I string theory is described by the N=1 supergravity (type I supergravity) in ten dimensions coupled to the SO(32) supersymmetric Yang-Mills theory. The discovery in 1984 by Michael Green and John Schwarz that anomalies in type I string theory cancel sparked the first superstring revolution.

In the 1990s it was realized that type I string theory with the string coupling constant g is equivalent to the SO(32) heterotic string with the coupling 1 / g. This equivalence is known as S-duality.