水线装甲是装在军舰的两侧的一种厚金属装甲，它可以装在船壳之外或是之内，一般常见于主力军舰如战列舰、战列巡洋舰、航空母舰和巡洋舰上。

一般来说战列舰上的主装甲带是从主甲板的高度起安装，一直到略低于水线为止。如果是安在船壳内侧的话还要有一定的倾角以提高防护性能。

Belt armor is a layer of heavy metal armor plated onto or within the outer hulls of warships, typically on battleships, battlecruisers and cruisers, and aircraft carriers.

The belt armor is designed to prevent projectiles from penetrating to the heart of a warship. When struck by an artillery shell or underwater torpedo, the belt armor either absorbs the impact and explosion with its sheer thickness and strength, or else uses sloping to redirect the projectile and its blast downwards.