  We present projected constraints on modified gravity models from the
observational technique known as 21 cm intensity mapping, where cosmic
structure is detected without resolving individual galaxies. The resulting map
is sensitive to both BAO and weak lensing, two of the most powerful
cosmological probes. It is found that a 200 m x 200 m cylindrical telescope,
sensitive out to z=2.5, would be able to distinguish DGP from most dark energy
models, and constrain the Hu & Sawicki f(R) model to |f_{R0}| < 9*10^(-6) at
95% confidence. The latter constraint makes extensive use of the lensing
spectrum in the nonlinear regime. These results show that 21 cm intensity
mapping is not only sensitive to modifications of the standard model's
expansion history, but also to structure growth. This makes intensity mapping a
powerful and economical technique, achievable on much shorter time scales than
optical experiments that would probe the same era.
