Although potassium is the eighth most abundant element on earth and comprises about 2.1% of the earth's crust, it is a very reactive element and is never found free in nature. Metallic potassium was first isolated by Sir Humphry Davy in 1807 through the electrolysis of molten caustic potash (KOH). A few months after discovering potassium, Davy used the same method to isolate sodium. Potassium can be obtained from the minerals sylvite (KCl), carnallite (KCl·MgCl₂·6H₂O), langbeinite (K₂Mg₂(SO₄)₃) and polyhalite (K₂Ca₂Mg(SO₄)₄·2H₂O). These minerals are often found in ancient lake and sea beds. Caustic potash, another important source of potassium, is primarily mined in Germany, New Mexico, California and Utah.

Pure potassium is a soft, waxy metal that can be easily cut with a knife. It reacts with oxygen to form potassium superoxide (KO₂) and with water to form potassium hydroxide (KOH), hydrogen gas and heat. Enough heat is produced to ignite the hydrogen gas. To prevent it from reacting with the oxygen and water in the air, samples of metallic potassium are usually stored submerged in mineral oil.

Potassium forms an alloy with sodium (NaK) that is used as a heat transfer medium in some types of nuclear reactors.

Potassium forms many important compounds. Potassium chloride (KCl) is the most common potassium compound. It is used in fertilizers, as a salt substitute and to produce other chemicals. Potassium hydroxide (KOH) is used to make soaps, detergents and drain cleaners. Potassium carbonate (KHCO₃), also known as pearl ash, is used to make some types of glass and soaps and is obtained commercially as a byproduct of the production of ammonia. Potassium superoxide (KO₂) can create oxygen from water vapor (H₂O) and carbon dioxide (CO₂) through the following reaction: 2KO₂ + H₂O + 2CO₂ => 2KHCO₃ + O₂. It is used in respiratory equipment and is produced by burning potassium metal in dry air. Potassium nitrate (KNO₃), also known as saltpeter or nitre, is used in fertilizers, match heads and pyrotechnics.