zh:常规 zh-hans:炮

海军或水军是一国军队当中的主要构成部分之一，为在海、河流、湖泊等水体上作战的军种，一般来说以锚作为标志。海军使用的武器主要为各式水上的军舰、以及在海底航行的潜艇；操控舰艇的海军为海军舰艇兵，除了在水上及海底作战的海军舰艇兵之外，海军还可衍生出空中战斗的海军航空兵、以及在水上与陆上战斗的海军陆战队。

海军是人类史上最古老的军种之一，在汉字中称为「水师」或「水军」，可以追溯到古代人类航海贸易、海上掠夺与自卫的发展，如西元前1400年的腓尼基人为保护于叙利亚沿岸穿越地中海创建殖民地和贸易活动，建造和使用了三列桨座战船于海上与敌人战斗。随着造船技术和工业革命的进步，海军船舰从人力划桨、风力推动的帆船到了轮机动力，航程大为增加；海军的武力也因为火砲的发明从水兵间的登舰近距离战斗演变到远距开火射击。第二次世界大战中，海军战斗模式丕变，巨砲厚甲的战舰逐渐退出世界舞台，取而代之的是潜艇和航空母舰空中武力的活跃，同时也在该时期爆发了史上最大规模的海上战役—雷伊泰湾海战。战后因为核动力能源的发明使得船舰航程趋近无限，武器也演变成具备长射程与精密性质的飞弹，海军的影响力也可延伸到更广的范围。

A navy or maritime force is a fleet of waterborne military vessels (watercraft) and its associated naval aviation, both sea-based and land-based. It is the branch of a nation's armed forces principally designated for naval and amphibious warfare; namely, lake-borne, riverine, littoral, or ocean-borne combat operations and related functions. It includes anything conducted by surface ships, amphibious ships, submarines, and seaborne aviation, as well as ancillary support, communications, training, and other fields; recent developments have included space-related operations. The strategic offensive role of a navy is projection of force into areas beyond a country's shores (for example, to protect sea-lanes, ferry troops, or attack other navies, ports, or shore installations). The strategic defensive purpose of a navy is to frustrate seaborne projection-of-force by enemies. The strategic task of the navy also may incorporate nuclear deterrence by use of Submarine-launched ballistic missiles. Naval operations can be broadly divided between riverine and littoral applications (brown-water navy), open-ocean applications (blue-water navy), and something in between (green-water navy), although these distinctions are more about strategic scope than tactical or operational division.