Abstract: Grain boundary (GB) deformation and twinning behavior have been recognized as important contributors to the plasticity of polycrystalline materials. However, a comprehensive understanding of dynamic interplay between GB deformation and twinning behavior remains largely elusive. Using in situ nanomechanical testing, we reveal that GB plasticity and twinning plasticity can be strongly coupled in the context of various deformation characteristics, including lamellae-type twinning from GBs, GB splitting-induced twinning, twinning from triple junctions (TJs), and GB-mediated hierarchical twinning. These GB/TJ-associated twinning modes often arise from the combined effect of macroscopic (geometry-dominated) and microscopic (excess volume-dominated) degrees of freedom of GBs/TJs as an effective way to alleviate local stress concentration, which in turn provides a chance of adjusting GB mobility and enhancing the coordinated evolution of entire interface network in three-dimensional space. Such coupling between GB plasticity and twinning plasticity should represent a general deformation mode in different metallic materials, holding important implications for preventing premature GB cracking and enhancing material ductility.