Tectonically uplifted ancient seawater was found within an exposed chalk formation (Avdat Group, Israel) of Eocene age at the foothills of the Judean Mountains. This seawater, along with its diluted derivatives, was found at elevations of 200–300 m above mean sea level (aMSL), significantly higher than the maximum flooding surfaces recorded since the Eocene. In contrast to modern-day seawater intrusion, which currently occurs along the Pleistocene alluvial fill of the Mediterranean coastal aquifer, the seawater trapped in the Avdat Formation shows no geochemical evidence of cation exchange. This is due to the absence of significant clay or organic material within the chalks. As a result, the trapped seawater has largely retained its original major ionic ratios. However, compared to standard seawater composition, it is enriched in Ca²⁺ and SO₄²⁻, suggesting that the intruding seawater dissolved gypsum deposited during Miocene transgression-regression cycles. A comparison with global records of ancient seawater compositions suggests that some of the trapped water dates back to the Eocene, indicating it is connate water (originating from the time of sedimentation). Other samples correspond to late Miocene seawater intrusions, representing interstitial water from later transgression-regression events. These seawater intrusions occurred prior to the final tectonic uplift that raised the region above sea level, fully exposing the Judean Mountains and their foothills. Tectonic reconstructions indicate that this seawater has been preserved since at least the Miocene, despite direct exposure to meteoric precipitation and the significant uplift that brought it to its current elevation (~250 m aMSL)—well above recorded maximum sea levels. This preservation is attributed to the dual-permeability characteristics of the chalk: low matrix permeability combined with high fracture permeability. This permeability structure channeled rainfall into underlying formations, allowing only partial dilution of the porewater and preventing significant flow since the Early Pliocene.