The present study investigated how increased right-hemisphere (RH) responses in healthy older adults during syntactic processing, a highly left-lateralized function in younger adults, are modulated by callosal integrity. Syntactic phrase violations in Taiwan Mandarin were presented to either the left or right visual field with event-related potentials (ERPs) measured. Individual participant’s microstructural tissue integrity of the corpus callosum was measured with diffusion tensor imaging (DTI). Younger and older adults elicited left-hemisphere (LH) P600 responses to grammatical violations, but only older adults additionally elicited a RH P600 grammaticality effect. Larger RH P600 effects in older adults were associated with larger fractional anisotropy (FA) values of the splenium. This association was not observed for the genu, which evinced significantly lower FA in older relative to younger adults. These results highlight that LH-similar syntactic responses are recruited in older adult RH even for the Mandarin language where syntactic information is routinely accessible via semantic cues. Critically, splenium integrity modulates how older individuals engage inter-hemispheric coordination for syntactic information processing.