井，是一种用来从地表下取水的设备，中国传说是伯益发明了井。

远古时代即有水井，《易经》记“改邑不改井”。孔颖达疏：“古者穿地取水，以瓶引汲，谓之为井。”在西元前721年，伊朗有坎儿井。文震亨的《长物志·凿井》介绍了当时凿井、用水以及供井神的情况：“泉井水味浊，不可供烹煮；然浇花洗竹，涤砚拭几，俱不可缺。凿井须于竹树下，深见泉脉，上置轱辘引汲，不则盖一小亭覆之。石栏古号‘银床’，取旧制最大而有古朴者置其上。井有神，井旁可置顽石，凿一小龛，遇岁时奠以清泉一杯，亦自有致”。

吴地信仰井神，称“井泉童子”。

A water well is an excavation or structure created in the ground by digging, driving, boring, or drilling to access groundwater in underground aquifers. The well water is drawn by a pump, or using containers, such as buckets, that are raised mechanically or by hand. Wells were first constructed at least eight thousand years ago and historically vary in construction from a simple scoop in the sediment of a dry watercourse to the stepwells of India, the qanats of Iran, and the shadoofs and sakiehs of India. Placing a lining in the well shaft helps create stability and linings of wood or wickerwork date back at least as far as the Iron Age.