FictionJunction YUUKA is a Japanese pop music unit formed as part of composer Yuki Kajiura's solo project, FictionJunction. The duo consists of vocalist Yuuka Nanri and composer, lyricist, and keyboardist Yuki Kajiura, who also serves as the producer for all of their recordings. The name FictionJunction YUUKA specifically represents the collaboration between Kajiura and Nanri, following the naming convention of FictionJunction followed by the vocalist's name. The pair first met when both were involved with the Minami Aoyama Shōjo Kagekidan, a stage group for teenage girls where Kajiura wrote music for plays and Nanri performed.

The musical identity of FictionJunction YUUKA is defined by Kajiura's signature compositional style, which blends pop structures with orchestral elements, piano-driven melodies, and choral arrangements. All songs are written, composed, and arranged by Yuki Kajiura, with Nanri providing the lead vocals. The duo's sound is closely tied to anime soundtracks, with most of their releases serving as theme songs or insert songs for television series. Their career began with their first recording, Akatsuki no Kuruma, an insert song for the mecha anime Mobile Suit Gundam SEED. Despite this being their first recording, their official debut single was Hitomi no Kakera, released on May 8, 2004, which served as the opening theme for the anime Madlax. The follow-up single, Inside Your Heart, was released on July 27, 2004, as the ending theme for the same series.

Due to the popularity of Akatsuki no Kuruma and the first anniversary of Gundam SEED, the song was released as a single on September 22, 2004, peaking at number ten on the Oricon weekly charts and selling over nineteen thousand copies in its first week. The duo continued their association with the Gundam franchise with Honō no Tobira, an insert song for Mobile Suit Gundam SEED Destiny released on September 22, 2005. This single made history as the first voice actor single to reach number one on the Oricon daily rankings. On November 23, 2005, FictionJunction YUUKA released their debut album, Destination, which reached number nine on the Oricon weekly charts.

Further anime work includes Silly-Go-Round, the opening theme for.hack//Roots, released on May 10, 2006. In the same year, they recorded Aikoi for the Tsubasa Chronicle anime series, which was included in the series fourth original soundtrack rather than released as a single. On November 22, 2006, they released Kōya Ruten, the opening theme for Sunrise's Bakumatsu Kikansetsu Irohanihoheto. Their seventh single, Romanesque, became the ending theme for the Bee Train anime El Cazador de la Bruja, released on April 18, 2007. FictionJunction YUUKA held their first live concert, Premium Live 2007, on February 8 and February 15 of that year. Their second album, Circus, was released on July 4, 2007, which included their recent singles, B-sides, the track Aikoi, and four new songs, debuting at number ten on the Oricon Album Charts.

As part of the larger FictionJunction project, Nanri has performed alongside other vocalists including KAORI, KEIKO, WAKANA, YURIKO KAIDA, and ASUKA. She is credited as a vocalist on the FictionJunction album Everlasting Songs, released on February 25, 2009, performing the song Kioku no Mori, which served as the ending theme for the OVA Tsubasa Shunraiki. FictionJunction YUUKA was active from 2003, with later performances including Yuki Kajiura Live Vol. 11 elemental FictionJunction Yuuka 2Days Special in February 2014 and a performance at the FlyingDog 10th Anniversary Live Inu Fes. in February 2019. The duo's discography is released under the Victor Entertainment and FlyingDog labels. Their industry significance is marked by their Oricon chart achievements, particularly Honō no Tobira becoming the first voice actor single to reach number one on the daily rankings, establishing a milestone for anime music recordings.