Hanja (Korean: 한자; Hanja: 漢字; IPA: [ha(ː)ntɕ͈a]), alternatively spelled Hancha, are Chinese characters used to write the Korean language. After characters were introduced to Korea to write Literary Chinese, they were adapted to write Korean as early as the Gojoseon period.

Hanjaeo (한자어; 漢字語) refers to Sino-Korean vocabulary, which can be written with Hanja, and hanmun (한문; 漢文) refers to Classical Chinese writing, although Hanja is also sometimes used to encompass both concepts. Because Hanja characters have never undergone any major reforms, they more closely resemble traditional Chinese and traditional Japanese characters, although the stroke orders for certain characters are slightly different. Such examples are the characters 教 and 敎, as well as 研 and 硏. Only a small number of Hanja characters were modified or are unique to Korean, with the rest being identical to the traditional Chinese characters. By contrast, many of the Chinese characters currently in use in mainland China, Malaysia and Singapore have been simplified, and contain fewer strokes than the corresponding Hanja characters.

Until the contemporary period, Korean documents, history, literature and records were written primarily in Literary Chinese using Hanja as its primary script. As early as 1446, King Sejong the Great promulgated Hangul (also known as Chosŏn'gŭl in North Korea) through the Hunminjeongeum. It did not come into widespread official use until the late 19th and early 20th century. Proficiency in Chinese characters is, therefore, necessary to study Korean history. Etymology of Sino-Korean words is reflected in Hanja.

Hanja were once used to write native Korean words, in a variety of systems collectively known as idu, but, by the 20th century, Koreans used hanja only for writing Sino-Korean words, while writing native vocabulary and loanwords from other languages in Hangul, a system known as mixed script. By the 21st century, even Sino-Korean words are usually written in the Hangul alphabet, with the corresponding Chinese character sometimes written next to it to prevent confusion if there are other characters or words with the same Hangul spelling. According to the Standard Korean Language Dictionary published by the National Institute of Korean Language (NIKL), approximately half (50%) of Korean words are Sino-Korean, mostly in academic fields (science, government, and society). Other dictionaries, such as the Urimal Keun Sajeon, claim this number might be as low as roughly 30%.

There is traditionally no accepted date for when literary Chinese (한문; 漢文; hanmun) written in Chinese characters (한자; 漢字; hanja) entered Korea. Early Chinese dynastic histories, the only sources for very early Korea, do not mention a Korean writing system. During the 3rd century BC, Chinese migrations into the peninsula occurred due to war in northern China and the earliest archaeological evidence of Chinese writing appearing in Korea is dated to this period. A large number of inscribed knife money from pre-Lelang sites along the Yalu River have been found. A sword dated to 222 BC with Chinese engraving was unearthed in Pyongyang.

From 108 BC to 313 AD, the Han dynasty established the Four Commanderies of Han in northern Korea and institutionalized the Chinese language. According to the Samguk sagi, Goguryeo had hanmun from the beginning of its existence, which starts in 37 BC. It also says that the king of Goguryeo composed a poem in 17 BC. The Gwanggaeto Stele, dated to 414, is the earliest securely dated relic bearing hanmun inscriptions. Hanmun became commonplace in Goguryeo during the 5th and 6th centuries and according to the Book of Zhou, the Chinese classics were available in Goguryeo by the end of the 6th century. The Samguk sagi mentions written records in Baekje beginning in 375 and Goguryeo annals prior to 600. Japanese chronicles mention Baekje people as teachers of hanmun. According to the Book of Liang, the people of Silla did not have writing in the first half of the 6th century but this may have been only referring to agreements and contracts, represented by notches on wood. The Bei Shi, covering the period 386–618, says that the writing, armour, and weapons in Silla were the same as those in China. The Samguk sagi says that records were kept in Silla starting in 545.

Some western writers claimed that knowledge of Chinese entered Korea with the spread of Buddhism, which occurred around the 4th century. Traditionally Buddhism is believed to have been introduced to Goguryeo in 372, Baekje in 384, and Silla in 527.

Another major factor in the adoption of hanmun was the adoption of the gwageo, copied from the Chinese imperial examination, open to all freeborn men. Special schools were set up for the well-to-do and the nobility across Korea to train new scholar officials for civil service. Adopted by Silla and Goryeo, the gwageo system was maintained by Goryeo until after the unification of Korea at the end of the nineteenth century. The scholarly elite began learning the hanja by memorizing the Thousand Character Classic (천자문; 千字文; Cheonjamun), Three Character Classic (삼자경; 三字經; Samja Gyeong) and Hundred Family Surnames (백가성; 百家姓; Baekga Seong). Passage of the gwageo required the thorough ability to read, interpret and compose passages of works such as the Analects (논어; 論語; Non-eo), Great Learning (대학; 大學; Daehak), Doctrine of the Mean (중용; 中庸; Jung-yong), Mencius (맹자; 孟子; Maengja), Classic of Poetry (시경; 詩經; Sigyeong), Book of Documents (서경; 書經; Seogyeong), Classic of Changes (역경; 易經; Yeokgyeong), Spring and Autumn Annals (춘추; 春秋; Chunchu) and Book of Rites (예기; 禮記; Yegi). Other important works include the Art of War (손자병법; 孫子兵法; Sonja Byeongbeop), Selections of Refined Literature (문선; 文選; Munseon), etc.