Electroless deposition (ED) or electroless plating is a chemical process by which metals and metal alloys are deposited onto a surface. Electroless deposition uses a chemical reaction that causes a metal to precipitate and coat nearby surfaces. The term "electroless" distinguishes this chemical process from electroplating, which uses an external electrical current to deposit metal onto surfaces. Electroless deposition thus can occur on non-conducting surfaces, making it possible to coat diverse materials including plastics, ceramics, and glass, etc. ED produced films can be decorative, anti-corrosive, and conductive. Common applications of ED include films and mirrors containing nickel and/or silver.

Electroless deposition changes the mechanical, magnetic, internal stress, conductivity, and brightening of the substrate. The first industrial application of electroless deposition by the Leonhardt Plating Company has flourished into metallization of plastics, textiles, prevention of corrosion, and jewelry. The microelectronics industry uses ED in the manufacturing of circuit boards, semi-conductive devices, batteries, and sensors.

Electroplating is generally cheaper than ED. Unlike ED, electroplating only deposits on other conductive or semi-conductive materials. Requiring an applied current, the instrumentation for electroplating is more complex. Electroless deposition deposits metals onto 2D and 3D structures, whereas other plating methods such as Physical vapor deposition (PVD), Chemical Vapor Deposition (CVD) are limited to 2D surfaces. Electroless deposition is advantageous in comparison to PVD, CVD, and electroplating deposition methods because it can be performed at ambient conditions. Electroless deposition can also produce very conformal coatings on complex structures like porous membranes.

The earliest known development of electroless plating was by the Moche culture of northern Peru around 100-500 CE. The Moche developed an electrochemical replacement process that deposited thin layers of gold (1-2 microns) onto copper objects without the use of an external electrical current. The process involved dissolving gold into a chemical solution using naturally occurring minerals such as ferric sulfate and alum, then adjusting the pH to prevent excessive corrosion before dipping copper objects into the solution. Gold ions spontaneously deposited onto the copper surface through an auto-catalytic chemical reaction. This technique was used extensively for decorative and ceremonial objects and predated the Western discovery of electroless plating by approximately 1,700 years.

In the Western world, the discovery of electroless deposition is attributed to Charles Wurtz who, in 1846, noticed a nickel-phosphorus solution spontaneously formed a black powder. 70 years later François Auguste Roux rediscovered the electroless deposition process and patented it in United States as the 'Process of producing metallic deposits'. Roux deposited nickel-phosphorus (Ni-P) onto a substrate, using electroless deposition, but his invention went uncommercialized. In 1946 the process was re-discovered by Abner Brenner and Grace E. Riddell while at the U.S. National Bureau of Standards. They presented their discovery at the 1946 Convention of the American Electroplaters' Society (AES); a year later, at the same conference they proposed the term "electroless" for the process and described optimized bath formulations, that resulted in a patent. However, neither Abner nor Riddell benefited financially from the filed patent. Deposition of Ni-P was commerciallized by Leonhardt Plating Company in Cincinnati followed by the Kannigen Co. Ltd in Japan, with revolutionary impact. The Leonhardt company designed and patented several deposition baths including plating of metals such as Pt, Sn, Ag, and their alloys.

The Tollens' reaction is often used in scientific demonstrations of ED. Tollen's reagent deposits a reflective metallic silver layer on glass, and is used for silvering mirrors. This reaction was once used to test for aldehydes in a basic solution of silver nitrate.

The ED process can be analyzed as four steps:

A typical electroless deposition bath consists of many components: