The Kōjien (広辞苑^?, "Wide garden of words") is a single-volume Japanese dictionary first published by Iwanami Shoten in 1955. Many native speakers of Japanese regard the Kōjien as the most authoritative dictionary, and newspaper editorials frequently cite its definitions. As of 2007, it has sold 11 million copies.

Kōjien was the magnum opus of Shinmura Izuru, 1876–1967, a professor of linguistics and Japanese at Kyoto University. He was born in Yamaguchi Prefecture and graduated from Japan's elite Tokyo University, where he was a student of Kazutoshi Ueda (上田万年^?, Ueda Kazutoshi, 1867–1937). After studying in Germany, Ueda taught comparative linguistics and edited foreign-language dictionaries in the latter part of the Meiji era. Through his tutelage, Shinmura became involved in Japanese language lexicography. Even Kōjien editions published after his death credit Shinmura as the chief editor.

The predecessor of the Kōjien originated during the Great Depression in East Asia. In 1930, the publisher Shigeo Oka (岡茂雄, Oka Shigeo, 1894–1989) wanted to create a Japanese dictionary for high school students. He asked his friend Shinmura to be chief editor, and they chose the title Jien (辞苑 "Garden of words") in a classical allusion to the Ziyuan (字苑, "Garden of characters") Chinese dictionary. Shinmura appointed his son Takeshi Shinmura (新村猛, Shinmura Takeshi, 1905–1992) as an editor, and in 1935, Hakubunkan (博文館) published the Jien dictionary. It contained some 160,000 headword entries of old and new Japanese vocabulary, as well as encyclopedic content, and quickly became a bestseller. The editors began working on a revised edition, but the 1945 Firebombing of Tokyo destroyed their work. After the war, Shinmura and his lexicographers began anew in September 1948. Iwanami Shoten published the first Kōjien in 1955.