Hanja (Hangul: 한자; Hanja: 漢字, Korean pronunciation: [ha(ː)nt͈ɕa]), alternatively known as Hancha, are Chinese characters (Chinese: 漢字; pinyin: hànzì) used in the writing of Korean. Hanja was used as early as the Gojoseon period, the first ever Korean kingdom.

Hanja-eo (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 kyūjitai and traditional Chinese characters, although the stroke orders for certain characters are slightly different. For example, 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.

Although a phonetic Hangul (also known as Chosŏn'gŭl in North Korea) had been created by Sejong the Great in 1446 through the promulgation of the Hunminjeongeum, it did not come into widespread official use until the late 19th and early 20th century. Thus, until that time it was necessary to be fluent in reading and writing Hanja to be literate in Korean, as Korean documents, history, literature and records throughout its history until the contemporary period were written primarily in Literary Chinese using Hanja as its primary script. Therefore, a good working knowledge of Chinese characters is still important for anyone who wishes to interpret and study older texts from Korea, or anyone who wishes to read scholarly texts in the humanities. A high proficiency in Hanja is also useful for understanding the etymology of Sino-Korean words as well as to enlarge one's Korean vocabulary.

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. By the 21st century, even Sino-Korean words are written in the Hangul alphabet most of the time, 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%.