Lateral connectivity between rivers and terrestrial landscapes is critical for both river and landscape health. Due to widespread anthropogenic degradation of riverscapes, river management is aiming to connect rivers to floodplains, riparian zones, and wetlands, putting a spotlight on lateral connectivity. However, there is currently no consensus on how to conceptualize and study lateral connectivity in rivers across disciplines. Here, we review lateral connectivity between riverscapes and terrestrial landscapes. We focus on the natural sciences, considering hydrology, geomorphology, ecology and biogeochemistry, but also consider social connectivity and the management and restoration of lateral connectivity. We emphasize the importance of considering the bidirectional nature of lateral connectivity, operating both into and out of river channels and the balance between these directions. The resulting “lateral connectivity balance” provides a framework to understand natural spatial and temporal variability in connectivity. Anthropogenic impacts have swung the balance of lateral connectivity, enhancing the transport of materials into and through river networks while suppressing fluxes from rivers to adjacent landscapes. We conclude that further research at the interfaces between the aquatic and terrestrial components of riverscapes is critical to advance our conceptual understanding of river and catchment systems. We propose that such research should be framed within the paradigm of “rebalancing” lateral connectivity, explicitly recognizing the natural bidirectionality of laterally connecting processes, the significance of the hydrologic, geomorphic, and biologic functions they support, and the value to society of the ecosystem services and climate change resilience they provide.