海水入侵（seawater intrusion）亦称盐水入侵（salt water intrusion），俗称海水倒灌，是指海水经地表到达陆地的水文现象，普遍出现于沿海低漥地区。其成因主要是与地层下陷与潮汐有关。台风季节或豪雨时亦很容易引发海水倒灌。

海水倒灌现象一般只在内河入海口处发生，可引发咸潮现象，威胁河流淡水水质。随着全球海平面的上升，海水倒灌势必越来越严重，所带来的可能是沿海地区土壤恶化腐蚀和淡水盐碱化等问题。

Saltwater intrusion is the movement of saline water into freshwater aquifers, which can lead to contamination of drinking water sources and other consequences. Saltwater intrusion occurs naturally to some degree in most coastal aquifers, owing to the hydraulic connection between groundwater and seawater. Because saltwater has a higher mineral content than freshwater, it is denser and has a higher water pressure. As a result, saltwater can push inland beneath the freshwater. Certain human activities, especially groundwater pumping from coastal freshwater wells, have increased saltwater intrusion in many coastal areas. Water extraction drops the level of fresh groundwater, reducing its water pressure and allowing saltwater to flow further inland. Other contributors to saltwater intrusion include navigation channels or agricultural and drainage channels, which provide conduits for saltwater to move inland, and sea level rise. Saltwater intrusion can also be worsened by extreme events like hurricane storm surges.