Tap water (running water, city water, town water, municipal water, sink water, etc.) is water supplied to a tap (valve). Its uses include drinking, washing, cooking, and the flushing of toilets. Indoor tap water is distributed through "indoor plumbing", which has existed since antiquity but was available to very few people until the second half of the 19th century when it began to spread in popularity in what are now developed countries. Tap water became common in many regions during the 20th century, and is now lacking mainly among people in poverty, especially in developing countries.

The faucet, commonly known as the water tap, is a very simple and yet sophisticated device which dates as far back as 1700 B.C. According to plumbing company Plumbing Help Today, when it was first invented, the faucet was used to control the water flowing to fountains in private homes and in public baths in ancient Rome.The idea of having fountains and baths was inspired by ancient Egyptians and Greeks who made valves to divert water from rivers and other water sources for drinking and irrigation.

Originally, the water tap had two handles; one for cold water and the other one for hot water. If someone wanted a warm bath they’d have to turn on both handles. This was the norm until 1937 when Alfred Moen, an American inventor and mechanical engineer, nearly burnt his hands trying to wash up.

Tap water is often culturally assumed to be drinking water, especially in developed countries. Usually it is potable, although water quality problems are not rare. Household water purification methods such as water filters, boiling, or distillation can be used when tap water's potability is doubted. The application of technologies (such as water treatment plants) involved in providing clean water to homes, businesses, and public buildings is a major subfield of sanitary engineering.