"Hare Hare Yukai" (ハレ晴レユカイ; lit. "Sunny Sunny Delight") is the ending theme song to the 2006 anime adaptation of The Melancholy of Haruhi Suzumiya (the first book in the Haruhi Suzumiya light novel series by Nagaru Tanigawa and several other short stories). The song is performed by the characters of the series: Haruhi Suzumiya (voiced by Aya Hirano), Yuki Nagato (voiced by Minori Chihara), and Mikuru Asahina (voiced by Yuko Goto). It was released as a physical single of the same title on May 10, 2006 by Lantis with the B-side "Welcome Unknown", the ending theme song to the radio show The Melancholy of Haruhi Suzumiya: SOS Brigade Radio Division that was also performed by the same actresses. "Hare Hare Yukai" has also appeared as a song in The Melancholy of Haruhi Suzumiya Character Song CD series, of which each character has their own solo version of the song, performed by their respective voice actor in the series.

"Hare Hare Yukai" generated public interest for the dance sequence in the ending animation, which was produced by Kyoto Animation and directed by Yutaka Yamamoto. Known colloquially as the "Haruhi dance", many people replicated the choreography, and it was credited for starting the cover dance video genre on video-sharing websites, particularly Nico Nico Douga and YouTube. The popularity of the song also led to growing interest in the Japanese voice acting industry, a phenomenon known as the "voice actor boom." The single was certified Gold by the Recording Industry Association of Japan.

"Hare Hare Yukai" is the ending theme song from the 2006 anime series The Melancholy of Haruhi Suzumiya (the animated adaptation of the first book and several short stories in the Haruhi Suzumiya series by Nagaru Tanigawa). It is composed by Tomokazu Tashiro [ja] and arranged by Takahiro Ando [ja]. The lyrics were written by Aki Hata. The vocals are performed by voice actresses Aya Hirano, Minori Chihara, and Yuko Goto, who portray the characters Haruhi Suzumiya, Yuki Nagato, and Mikuru Asahina respectively. "Hare Hare Yukai" was released as a physical single of the same title on May 10, 2006 by Lantis. The B-side of the single, "Welcome Unknown", also performed by the same actresses, was used as the ending theme song to The Melancholy of Haruhi Suzumiya: SOS Brigade Radio Division, a promotional radio show for The Melancholy of Haruhi Suzumiya.

During the broadcast of The Melancholy of Haruhi Suzumiya, Lantis released The Melancholy of Haruhi Suzumiya Character Song CD, a series of character song CDs, for each major character of the series: Haruhi Suzumiya, Yuki Nagato, Mikuru Asahina, Tsuruya (voiced by Yuki Matsuoka), Ryoko Asakura (voiced by Natsuko Kuwatani), Kyon's sister (voiced by Sayaka Aoki), Emiri Kimidori (voiced by Yuri Shiratori), Itsuki Koizumi (voiced by Daisuke Ono), and Kyon (voiced by Tomokazu Sugita). Each character CD included a solo version of "Hare Hare Yukai", some with different lyrics and arrangements.

The ending animation of The Melancholy of Haruhi Suzumiya, created by Kyoto Animation, featured the main characters dancing, of which the choreography became colloquially known as the "Haruhi dance." The ending animation was directed and storyboarded by Yutaka Yamamoto. Yamamoto stated that he was inspired by the ending theme song "Fun Fun & Shout" from the 2002 OVA series Haré+Guu Deluxe, which also featured a dance sequence in its animation. Anthropologist Ryotaro Mihara observed that several parts of the choreography featured in the dance took inspiration from dance routines from the Japanese girl group Berryz Kobo. Shoko Ikeda, who worked on the ending animation, stated that she had no dance experience and that she saw a "glimpse of hell" as soon as the word "dance" was mentioned. She said that the only challenge she faced was having to watch idol music videos for reference.

On September 16, 2006, Kyoto Animation announced on their website that due to popular demand, they would release Yamamoto's storyboard of the full choreography on their website later in the month. Aya Hirano, Minori Chihara, and Yuko Goto, the voice actresses of Haruhi, Yuki, and Mikuru, revealed the choreography to the full version of "Hare Hare Yukai" to the public when they performed the song at the 2006 Animelo Summer Live. A fully animated choreography video featuring the TV size version of "Hare Hare Yukai" was included as a bonus extra on vol. 7 of the DVD home release of The Melancholy of Haruhi Suzumiya, which went on sale on January 26, 2007.

The physical single of "Hare Hare Yukai" was sold out at stores in Akihabara, Japan on the first day of release, with Oricon reporting 34,881 copies sold on its first week. By July 2006, it sold over 100,000 copies and earned a Gold certification from the Recording Industry Association of Japan. In the same year, "Hare Hare Yukai" won the Theme Song Award at the 11th Animation Kobe Awards. At Anime Expo 2008, the Society for the Promotion of Japanese Animation awarded "Hare Hare Yukai" with Best Original Song. In 2019, "Hare Hare Yukai" won the Audience Choice Award at the Heisei Anisong Grand Prix for the decade of 2000 to 2009.

Hirano, Chihara, and Goto first performed "Hare Hare Yukai" at the 2006 Animelo Summer Live. On March 18, 2007, they performed the song again with Tomokazu Sugita (the voice actor of Kyon) and Daisuke Ono (the voice actor of Itsuki Koizumi) at The Extravagance of Haruhi Suzumiya, a live concert event for the cast of The Melancholy of Haruhi Suzumiya.