洞里萨湖（高棉语：បឹងទន្លេសាប，罗马化：Tonlé Sap；意为「巨大的淡水湖」或纯粹「大湖」）或洞里湖，又名金边湖，位于柬埔寨境内西部，是东南亚最大的淡水湖。

枯水期（12月－4月）湖水经洞里萨河注入湄公河，湖水平均深度为1米，面积2,700平方千米；雨季因湄公河回流，水深可达9米，面积则扩展至16,000平方千米。

因为湄公河水内含冲积物质所带来的养份，使得湖中滋生大量的鱼虾，无论旱、雨季都出产甚丰。目前洞里萨湖是柬埔寨北部的主要「肉食仓库」，湖的周围有三百万以上人民直接或间接地以渔业为生。

Tonlé Sap (Khmer:ទន្លេសាប IPA: [tunleː saːp], literally large river (tonle); fresh, not salty (sap), commonly translated to "Great Lake") refers to a seasonally inundated freshwater lake, the Tonlé Sap Lake and an attached river, the 120 km (75 mi) long Tonlé Sap River, that connects the lake to the Mekong. They form the central part of a complex hydrological system, situated in the 12,876 km (4,971 sq mi) Cambodian floodplain covered with a mosaic of natural and agricultural habitats that the Mekong replenishes with water and sediments annually. The central plain formation is the result of millions of years of Mekong alluvial deposition and discharge. From a geological perspective, the Tonlé Sap Lake and Tonlé Sap River are a current freeze-frame representation of the slowly, but ever shifting Lower Mekong Basin. Annual fluctuation of the Mekong's water volume, supplemented by the Asian Monsoon regime causes the unique flow reversal of the Tonle Sap River.