River deltas, where ocean tides can often freely propagate into the river, are focal points of human settlement. Beneath the delta surface, groundwater stored in the soil strata fluctuates with river tides, resulting in pressure variations within the soil strata. Here, we introduce a novel ultra‐weak fiber‐optic instrument to test the hypothesis that tide‐induced groundwater level variations induce periodic soil stratum deformation. Using borehole deployments ranging over 70 m in the subsurface of the Yangtze Delta, we observe semidiurnal and spring–neap cycles of soil stratum deformation and reveal its dependence on lithology and depth. This allows us to coin the new term “soil stratum tide,” defined as the periodic deformation of soil strata in response to pressure fluctuations induced by tides in open water. High‐accuracy monitoring of the breathing of tidal deltas enables calibrating Earth observation systems, analysis of delta subsidence, and safeguarding of infrastructure jeopardized by soil stratum deformation. Plain Language Summary River deltas are important population centers, and deformation of their subsurface endangers the safety of nearly half a billion people worldwide. In tidal deltas, groundwater stored in soil strata fluctuates with river tides, resulting in pressure changes within the soil strata. Whether these pressure variations translate into expansion–shrinking cycles of the soil strata remains unknown to date. We break new ground in this field by developing a fiber‐optic instrument with microstrain accuracy and meter‐scale resolution. Using this novel sensing approach, we observe tidal soil stratum deformations beneath the Yangtze Delta and evaluate their dependence on lithology and depth. We coin the new term “soil stratum tide”, and pave the way for high‐accuracy deformation monitoring of the shallow subsurface of river deltas. Key Points We develop a novel quasidistributed ultra‐weak fiber Bragg grating instrument to observe and measure subtle subsurface deformation Our monitoring data provide the first view on soil stratum tidal deformation to date The energy of soil stratum tides is dominantly controlled by subsoil stiffness, permeability, and pore pressure tidal amplitude