Cornelius (born Keigo Oyamada (小山田圭吾 Oyamada Keigo) January 27, 1969 in Setagaya, Tokyo, Japan) is a Japanese recording artist and producer.

He was ranked at number 43 in a list of Japan's top 100 musicians by VHM.

Oyamada's first claim to fame was as a member of the pop duo Flipper's Guitar, one of the key groups of the Tokyo Shibuya-kei scene. Following the disbandment of Flipper's Guitar in 1991, Oyamada donned the "Cornelius" moniker and embarked on a successful solo career.

American music journalists often describe Cornelius's musical style as being similar to Beck's, whom he acknowledges as an influence along with The Beatles, The Beach Boys, The Jesus and Mary Chain, Primal Scream and the Brazilian band Kassin + 2, among others.

The music of Cornelius could be described as experimental and exploratory, and often incorporates dissonant elements alongside more familiar harmonically "pleasing" sounds. This tension, plus his practice of bringing in sounds and samples from mass culture, pure electronic tones, and sounds from nature (such as on his Point album), lead him to being sometimes categorized as an "acquired taste."

He chose his pseudonym in tribute to the character of the same name from the movie Planet of the Apes. He commissioned a song, about himself, on Momus' 1999 album Stars Forever.

Oyamada married fellow musician and collaborator Takako Minekawa in 2000 and they have one child, Milo, named after the son of Cornelius in Planet of the Apes. He is a second cousin of Joi Ito.

His song Wataridori 2 is available for download via the Creative Commons CD

As of September 2006, he was no longer signed to Matador Records.

In 2006 and 2007 respectively, the song "The Micro Disneycal World Tour" from the "Fantasma" album, was used for Nick Parks' "Creature Comforts" and Sky television's "See, Surf, Speak" advertisements in the UK.