Activated carbon, also called activated charcoal, is a form of carbon commonly used to filter contaminants from water and air, among many other uses. It is processed (activated) to have small, low-volume pores that greatly increase the surface area available for adsorption or chemical reactions. (Adsorption, not to be confused with absorption, is a process where atoms or molecules adhere to a surface). The pores can be thought of as a microscopic "sponge" structure. Activation is analogous to making popcorn from dried corn kernels: popcorn is light, fluffy, and its kernels have a high surface-area-to-volume ratio. Activated is sometimes replaced by active.

Because it is so porous on a microscopic scale, one gram of activated carbon has a surface area of over 3,000 square metres (32,000 square feet), as determined by gas absorption and its porosity can run 10ML/day in terms of treated water per gram. Researchers at Cornell University synthesized an ultrahigh surface area activated carbon with a BET area of 4,800 m² (52,000 sq ft). This BET area value is the highest reported in the literature for activated carbon to date. For charcoal, the equivalent figure before activation is about 2–5 square metres (22–54 sq ft). A useful activation level may be obtained solely from high surface area. Further chemical treatment often enhances adsorption properties.

Activated carbon is usually derived from waste products such as coconut husks in addition to other agricultural wastes like olive stones, rice husks and nutshell shells which are also being upcycled into activated carbon, diversifying feedstock supply. Furthermore, waste from paper mills has been studied as a possible source of activated carbon. These bulk sources are converted into charcoal before being activated. Using waste streams not only reduces landfill burden but also works to lower the overall carbon footprint of activated carbon production as previously discarded waste is now repurposed. When derived from coal, it is referred to as activated coal. Activated coke is derived from coke. In activated-coke production, the raw coke (most commonly petroleum coke) is ground or pelletized, then "activated" via physical (steam or CO₂ at high temperature) or chemical (e.g., KOH or H₃PO₄) methods to introduce a porous network, yielding a high-surface-area adsorbent which is referred to as activated coal.

Activated carbon is used in methane and hydrogen storage, air purification, capacitive deionization, supercapacitive swing adsorption, solvent recovery, decaffeination, gold purification, metal extraction, water purification, medicine, sewage treatment, air filters in respirators, filters in compressed air, teeth whitening, production of hydrogen chloride, edible electronics, and many other applications. These multiuse applications make it a versatile form of carbon that is used daily in many industries.

There are many industrial applications of activated carbon and its other forms such as areas like metal extraction, water purification, sewage treatment, metal finishing and more. For example, it is the main purification technique for removing organic impurities from bright nickel plating solutions used for metal finishing plants. Expanding on electroplating, a variety of organic chemicals can be added to the plating solutions for improving their deposit qualities and for enhancing properties like brightness, smoothness, ductility, etc. This is due to the passage of direct current and electrolytic reactions of anodic oxidation and cathodic reduction, organic additives generate unwanted breakdown products in solution. Their excessive build up in the solutions can adversely affect plating quality and physical properties of deposited metal if run untreated by the filters. Activated carbon treatment removes such impurities and restores plating performance to the desired level. Its installation costs may vary according to the volume of water it must process, however the average cost can be around USD 1 - 2 million. Additionally, these filters need replacing over time (typically 6-12 months depending on usage). The cost of replacing the carbon in the GAC filter form is about USD 0.05 - 0.1 per cubic meter of water that is treated in the plant.

Activated carbon is used to treat poisonings and overdoses following oral ingestion. Tablets or capsules of activated carbon are used in many countries as an over-the-counter drug to treat diarrhea, indigestion, and flatulence. However, activated charcoal shows no effect on intestinal gas and diarrhea, is ordinarily medically ineffective if poisoning resulted from ingestion of corrosive agents, boric acid, or petroleum products, and is particularly ineffective against poisonings of strong acids or bases, cyanide, iron, lithium, arsenic, methanol, ethanol, or ethylene glycol. Activated carbon will not prevent these chemicals from being absorbed into the human body. It is on the World Health Organization's List of Essential Medicines.

Incorrect application (e.g. into the lungs) results in pulmonary aspiration, which can sometimes be fatal if immediate medical treatment is not initiated.

Activated carbon, in 50% w/w combination with celite, is used as stationary phase in low-pressure chromatographic separation of carbohydrates (mono-, di-, tri-saccharides) using ethanol solutions (5–50%) as mobile phase in analytical or preparative protocols.