Ōtsuki Fumihiko (大槻文彦, October 15, 1847; –February 17, 1928) was a Japanese lexicographer, linguist, and historian. He is best known for two Japanese-language dictionaries that he edited, Genkai (言海, literally "sea of words," 1891) and its successor Daigenkai (大言海, literally "great sea of words," 1932–1937), and for his studies of Japanese grammar.
Ōtsuki Fumihiko was born in the Kobiki-chō (木挽町) section of Edo in what is now part of Ginza, Tokyo. He was the third son of the Confucian scholar and gunnery expert Ōtsuki Bankei (大槻磐渓) and the grandson of the Confucian and Western scholar Ōtsuki Gentaku (大槻玄沢). Following family tradition, he embarked on Western studies, studying English and mathematics at the Kaiseijo school (one of the predecessors to the University of Tokyo). He later worked as a translator while learning English from Americans in the port city of Yokohama. Around 1872, he joined in the editing of an English-Japanese dictionary, and he later worked on textbooks and taught at schools in Miyagi Prefecture.
Although Ōtsuki paid for the original publication expenses for Genkai himself, it was soon republished and expanded in commercial editions that went through over a thousand printings. Modeled in part on Western monolingual dictionaries, Genkai gave not only basic information about words—their representations in kana and kanji and their definitions in Japanese—but also pronunciations and etymologies and citations of their use. Its successor, the four-volume Daigenkai, though published under Ōtsuki's name and based in part on his work, appeared some years after his death and was completed by other lexicographers.
Ōtsuki's grammatical works, especially Kō Nihon Bunten (広日本文典, "A Comprehensive Japanese Grammar") and Kōgohō (口語法, "The Grammar of Spoken Japanese"), had a strong influence on how Japanese grammar would be taught in Japanese schools for generations to come.