Doenjang (Korean: 된장; pronounced [tøn.dʑaŋ]; lit. 'thick sauce') or soybean paste is a type of fermented bean paste made of soybean and brine used in Korean cuisine. It is also a byproduct of soup soy sauce production. It is sometimes used as a relish.

The earliest soybean fermentations in Korea seem to have begun prior to the era of the Three Kingdoms. The Records of the Three Kingdoms, a Chinese historical text written and published in the third century AD, mentions that "Goguryeo people are good at brewing fermented soybeans" in the section named Dongyi (Eastern foreigners) in the Book of Wei. Jangdoks used for doenjang production are found in the mural paintings of Anak Tomb No. 3 from the 4th century Goguryeo.

In the Samguk sagi, a historical record of the Three Kingdoms of Korea, it is written that doenjang and ganjang, along with meju and jeotgal, were prepared for the wedding ceremony of King Sinmun in February 683. Sikhwaji, a section from Goryeosa (History of Goryeo), recorded that doenjang and ganjang were included in the relief supplies in 1018, after a Khitan invasion, and in 1052, when a famine occurred. Joseon texts such as Guhwangchwaryo and Jeungbo sallim gyeongje contain detailed procedures on how to brew good-quality doenjang and ganjang. Gyuhap Chongseo explains how to pick a date for brewing, what to forbear, and how to keep and preserve doenjang and ganjang.

Doenjang is made using fermented soybean and brine. Soup soy sauce is also made during the doenjang production.

Meju, Korean soybean brick, is made around ipdong in early November. Soybeans are soaked overnight, boiled in salt water, and then pounded in a mortar (jeolgu) or coarsely ground in a millstone. About a doe (about 1.8 L (110 cu in)) or two does of pounded soybean is chunked, compressed, and shaped into a cube or a sphere called meju. The meju bricks are then dried in a cool, shaded area for a week to several weeks until firm. When the bricks harden, they are tied with rice straws to the eaves of the house, or put in the warm ondol room with rice straws, for fermentation. In Jeongwol, the first month of the lunar year, well-fermented meju bricks are washed and sun-dried.

After drying, the meju bricks are aged in onggi crocks (jangdok) with brine. Charcoal and chillies are added for their absorbent and antibacterial properties, as well as folk-religious beliefs that they drive evil spirits away. When fermented well, the aged meju chunks are mashed to become doenjang, and the filtrate is boiled to become ganjang.

Though doenjang and ganjang are usually made together, doenjang can also be made without producing any filtrate.

While traditional doenjang is made with soybeans and brine only, many factory-made variants of doenjang contain a fair amount of wheat flour just like most factory-made soy sauce does. Some current makers also add fermented, dried, and ground anchovies to accentuate doenjang's savory flavor. Korean Ministry of Food and Drug Safety's Food Code classifies doenjang into three categories by their ingredients.