水圈，是由水构成的系统，泛指地表上所有的水，占了地表百分之七十的面积。

指地球上的水以气态、液态和固态三种形式存在于空中、地表和地下，包括大气水、海水、陆地水（河、湖、沼泽、冰雪、土壤水和地下水），以及生物体内的生物水；而我们通常所说的水圈一般是指地球上被冰雪、液态水和水汽所占据而构成的壳层。水圈的上限可视为对流层顶，下限为深层地下水所及的深度。

The hydrosphere (from Greek ὕδωρ - hydōr, "water" and σφαῖρα - sphaira, "sphere") in physical geography describes the combined mass of water found on, under, and over the surface of a planet.

Igor Shiklomanov, the man selected by the United Nations to do its world inventory of water resources, estimated that there are 1386 million cubic kilometres of water on Earth. This includes water in liquid and frozen forms in groundwater, oceans, lakes and streams. Glaciers, against popular belief, are part of the cryosphere and not part of the Hydrosphere. Saltwater accounts for 97.5% of this amount. Fresh water accounts for only 2.5%. Of this fresh water 68.7% is in the "form of ice and permanent snow cover in the Arctic, the Antarctic, and in the mountainous regions. Next, 29.9% exists as fresh groundwaters. Only 0.26% of the total amount of fresh waters on planet Earth is easily accessible. It is found in lakes, reservoirs and river systems, the principal elements of water ecosystems." The total mass of the Earth's hydrosphere is about 1.4 × 10 tonnes, which is about 0.023% of the Earth's total mass. About 20 × 10 tonnes of this is in the Earth's atmosphere (for practical purposes, 1 cubic metre of water weighs one tonne). Approximately 75% of the Earth's surface, an area of some 361 million square kilometers (139.5 million square miles), is covered by ocean. The average salinity of the Earth's oceans is about 35 grams of salt per kilogram of sea water (3.5%).