The original language of Japan was the so-called yamato kotoba. In addition to this original language, Japanese also has a great number of words that were either borrowed from Chinese or constructed on Chinese patterns. These words entered the language from the fifth century onwards via contact with Chinese culture. Chinese based words comprise as much as seventy percent of the total vocabulary of the Japanese language and form as much as thirty to forty percent of words used in speech.

A much smaller number of words has been borrowed from Korean and Ainu. Japan has also borrowed a number of words from other languages, gairaigo. This began with borrowings from Portuguese in the 16th century, followed by borrowing from Dutch during Japan's long isolation of the Edo period. With the Meiji restoration and the reopening of Japan in the 19th century, borrowing occurred from German, French and English. Currently, words of English origin are the most commonly borrowed.

In the Meiji era, the Japanese also coined many neologisms using Chinese patterns to translate Western concepts. The Chinese and Koreans imported many of these pseudo-Chinese words into Chinese, Korean, and Vietnamese via their kanji characters in the late 19th and early 20th century. For example, 政治 seiji ("politics"), and 化学 kagaku ("chemistry"). As a result, Japanese, Chinese, Korean, and Vietnamese share a large common corpus of vocabulary in the same way a large number of Greco-Roman words is shared among European languages.

In the past few decades, wasei-eigo (made-in-Japan English) has become a prominent phenomenon. Words such as wanpataan (< one + pattern, "to be in a rut", "to have a one-track mind") and sukinshippu (< skin + -ship, "physical contact"), although coined from English, are nonsensical in a non-Japanese context. A small number of such words have been borrowed back into English.

Additionally, many native Japanese words have become commonplace in English, due to the popularity of many Japanese cultural exports. Words such as sushi, judo, karate, sumo, karaoke, origami, samurai, haiku, ninja, sayonara, rickshaw (from 人力車 jinrikisha), futon, and many others have become part of the English language.