Konjaku Monogatarishū (今昔物語集; lit. Anthology of Tales Old and New), also known as the Konjaku Monogatari (今昔物語), is a Japanese collection of over one thousand tales written during the late Heian period (794–1185). The entire collection was originally contained in 31 volumes, of which 28 remain today. The volumes cover various tales from India, China and Japan. Detailed evidence of lost monogatari exist in the form of literary critique, which can be studied to reconstruct the objects of their critique to some extent.

Each tale in the Konjaku Monogatarishū starts with the phrase once upon a time (今は昔) (lit. now long ago), which in its Japanese reading is pronounced ima wa mukashi. The Sino-Japanese reading of this phrase is konjaku, and it is from the Chinese-style reading that the collection is named. The Konjaku Monogatarishū is commonly known by the shorter name "Konjaku Monogatari". Since it is an anthology rather than a single tale, however, the longer title is more accurate.

The Konjaku Monogatarishū is divided according to the region of the text. The first five volumes, the 天竺 (Tenjiku) section, contain tales set in India. The next five volumes, the 震旦 (Shintan) section, contain tales set in China. The remainder of the anthology, the 本朝 (Honchō) section, contains tales from Japan.

The arrangement of the stories is in parallel to how Buddhism travelled to Japan. The collection emphasizes the path that Buddhism took to Japan in order to further understand what Buddhism means to Japan. First, Buddhism leaves India and becomes very popular in China. As many things have been borrowed from the Chinese, Buddhism then travels to Japan. Each move leads to a morphing of the basics of this religion so by that time it arrived in Japan, it became a new form of Buddhism for the Japanese.

The subject-matter is largely drawn from Buddhist and popular folklore. The anthology contains no mythology, and references to Shinto-related themes are notably few. The Buddhist tales cover a wide range of topics; both historical tales about the development, transmission and spread of Buddhism along with dogmatic tales which emphasize karmic retribution. The folkloric tales mostly depict encounters between human beings and the supernatural. The typical characters are drawn from Japanese society of the time — nobility, warriors, monks, scholars, doctors, peasant farmers, fishermen, merchants, prostitutes, bandits, beggars, widows. Their supernatural counterparts are oni and tengu.

The work is anonymous. Several theories of authorship have been put forward: one argues that the compiler was Minamoto no Takakuni, author of Uji Dainagon Monogatari; another suggests the Buddhist monk Tobane Sōjō, and a third one proposes a Buddhist monk living somewhere in the vicinity of Kyoto or Nara during the late Heian period. So far no substantive evidence has emerged to decide the question and no general consensus has formed.

The date of the work is also uncertain. From the events depicted in some of the tales it seems likely that it was written down at some point during the early half of the 12th century, after the year 1120.

The oldest extant copy of the Konjaku Monogatarishū is the Suzuka Manuscript (鈴鹿家旧蔵本). Designated as a National Treasure in 1996, it was assembled by a Shinto priest named Tsuretane Suzuka in the Kamakura period (1185–1333). The manuscript was then brought to Kyoto University by a descendant who was a librarian at the university for donation and archiving. The manuscript has been scanned and made available in digital format on the internet.