"Hikari" (Japanese: 光, "light") is a song recorded by Japanese–American recording artist Utada Hikaru for her fourth studio and third Japanese language album, Deep River (2002). It premiered on March 20, 2002 as the third single from the album in Japan. It was written and composed by Utada, whilst production and arrangement was handled by Utada, her father Teruzane Utada, and long-time collaborator Miyake Akira. The single, and a remix by Russell McNamara (under the alias PlanitB), was used as the official Japanese theme song for the 2002 action role-playing video game Kingdom Hearts, and appeared on its original soundtrack respectively. Musically, "Hikari" is pop folk song. Lyrically, it is about mysteries in life and human activities.

Upon its release, the track garnered positive reviews from music critics. Many critics highlighted the track as one of Utada's best singles, and commended her vocal abilities and songwriting. It was also successful in Japan, peaking at number one both on the Oricon Singles Chart and Tokyo Broadcasting System's (TBS) Count Down TV singles chart. It was certified platinum by the Recording Industry Association of Japan (RIAJ) for physical shipments of 500,000 units. An accompanying music video was shot by her then-husband, Kazuaki Kiriya; it features Utada washing dishes and drinking water. It was performed on some of her concert tours, including the Utada United and Wild Life tour.

To promote the international formats of Kingdom Hearts, Utada re-recorded an English language version entitled "Simple & Clean". Both the original edit and remix version by PlanitB served as international theme songs. It did not appear on Utada's English studio Exodus (2004), but the original version was included on her 2009 English studio album This Is the One. It was later released as an A-side 12-inch single with Utada's single "Colors" in 2003, and received positive reviews from most music critics.

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