We report on the discovery of a novel diffusion pathway for silicon dimers on the Si(001) surface. As a small molecule, the dimer’s configuration can play a central role in its diffusion. Using scanning tunneling microscopy movies at temperatures near 450 K, we show, in real time, changes in the dimer’s configuration during its diffusion. These changes in configuration provide a pathway for diffusion across the substrate dimer rows, unlike the well-known diffusion along the dimer rows. The important energies involved in dimer row crossing are measured.