Hydration pack is a backpack or undershirt with a built-in water bag (also known as a hydration pouch) and a hose that makes it easy for the user to drink with their hands free while exercising. It is commonly used for hiking, cycling, running and other endurance sports.

The water bag is usually made of flexible plastic and can hold 1 to 3 liters of water. The attached hose extends to the shoulder and has a bite valve, making it easy for the user to drink without stopping or using their hands. Many water bags also feature additional compartments for small items such as snacks, keys or cell phones.

Water bags help people stay hydrated during prolonged physical activity, especially outdoors or in hot environments. They are convenient, comfortable and ensure easy access to water at all times, thus improving athletic performance.