Formed in Gunma in 2004, with Iyori Shimizu as the central figure, the band went through several member changes before reaching its current lineup in 2007.

On February 18th, 2009, they released their first mini album, "逃した魚" (The Fish That Got Lost). For the first time in history, they were simultaneously selected as Tower Records' "Tower Recommend" and HMV's "HOT PICKS," leading to an unprecedented large-scale distribution in stores nationwide. In June 2010, they released their long-awaited first full album, "あとのまつり" (The Festival After). They established themselves as a band with heartbreaking lyrics and beautiful melodies that set them apart from the rest.

Just before their debut, they were selected by iTunes as the new artist most likely to break out in 2011, "Japan Sound of 2011." Their major debut single, "はなびら," released on April 6th, reached number 45 on the Oricon Weekly Chart, and their second single, "花束," released on June 22nd, received 73 hits, including ending plays on FM radio stations nationwide and as the ending theme for TV programs, reaching number 18 on the Oricon Weekly Chart. They then released their third single, "思い出せなくなるその日まで" in October, followed by their first major album, "スーパースター" which reached number 4 "スーパースター" the Oricon Weekly Chart.

In 2011, their second single "花束" reached number one on the annual radio airplay chart for Japanese music, won the Gold Disc Award for Best 5 New Artist, and was awarded the CD Shop Award. Their solo tour of 11 locations across the country (including two additional shows) was sold out immediately, and their additional show at Shibuya Public Hall was also sold out immediately. Despite being a newcomer, they have also performed at six summer festivals: "ROCK IN JAPAN," "RISING SUN ROCK FES.", "MONSTER baSH," "SETSTOCK," "HIGHER GROUND," and "TREASURE05X."

In 2012, they released four singles: "恋," "日曜日," "わたがし," and "青い春." "日曜日" was used as the theme song for the HBC/TBS drama "Soup Curry," and "青い春" was used as the theme song for the CX drama "Koukou Nyushi," attracting even more attention. In November, they released their third album, "blues" which included these singles, and then embarked on a nationwide tour with 19 shows in 17 locations, all of which sold out on the day of their release. Furthermore, their first performance at the Nippon Budokan on September 7, 2013, was also a huge success, selling out on the day of their release.

In 2014, they released their new album "Love Story" on March 26th, which includes their 8th single "高嶺の花子さん" (released in June 2013), 9th single "fish"(released in February 2014) and 10th single "繋いだ手から" (released in March 2014), and has already sold over 120,000 copies!!! In May, they embarked on their first nationwide hall tour "Love Stories Tour 2014" (20 locations, 21 shows) to promote the album, all of which sold out on the same day!!! The additional shows, a 2-day concert at Yokohama Arena and a 1-day concert at Osaka-jo Hall, were also sold out on the same day, making for a huge success.

Their single "ヒロイン," released in January 2015, was used as the commercial song for JR SKISKI, "SISTER," released in May, was the commercial song for Otsuka Pharmaceutical's Pocari Sweat Ion Water, "手紙," released in August, was the commercial song for NTT Docomo, and "クリスマスソング," released in November, was used as the theme song for the Fuji TV Monday 9pm drama "5→9~My Buddhist Boyfriend~," both of which became huge hits. Their fifth original album, "シャンデリア," then topped the Oricon Weekly Chart for two consecutive weeks.

In January 2016 they, embarked on a hall tour of 27 shows at 26 locations, followed by an arena tour of 12 shows at 6 locations, all of which sold out immediately. Their single "僕の名前を" released in May, was the theme song for the film "Wolf Girl and Black Prince," reaching number one on the iTunes weekly chart, and his single "ハッピーエンド," released in November, attracted attention as the theme song for the film "I'll Date Tomorrow, with Yesterday's You." They then released their first greatest hits album, "アンコール" on December 28th, becoming a huge hit, selling over 800,000 copies.

Their largest nationwide arena tour to date, "All Our Yesterdays Tour 2017," which began in February 2017, attracted 300,000 people, and their song "瞬き," the theme song for the film "The 8-Year Engagement: A True Story of a Miracle," released in December 2017, became a best-seller. From July to October, they completed a dome tour with five shows in Tokyo, Nagoya, and Osaka, all of which were sold out, resulting in great success. Following the success of "オールドファッション," the theme song for the TBS Friday drama "Big Love: With You Who Forgets Me" in October 2018, and "HAPPY BIRTHDAY" the theme song for the TBS Tuesday drama "A Story to Read on the Day You Fall in Love for the First Time" in January 2019, their sixth original album "MAGIC" became a huge hit, selling over 500,000 copies, and their "NO MAGIC TOUR 2019," which began in April 2019, was a huge success, attracting 350,000 people across 35 performances.
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2020年8月には、コロナ禍で中止となったインターハイを目指していた高校生に向けて書き下ろした「水平線」が、中止になった“令和2年度インターハイ”応援ソングに決定し、10代を中心にSNSで拡散され大きく話題となった。
10月にはTBS日曜劇場「危険なビーナス」主題歌の「エメラルド」を配信シングルとしてリリース。
2021年、back number初となる楽曲提供でBTSの新曲「Film out」の楽曲制作とプロデュースを担当。日本のみならず世界中から注目を集めた。
4月に配信リリースされた「怪盗」は、日本テレビ系 水曜ドラマ「恋はDeepに」主題歌に起用され大ヒットを記録。
8月、前年よりYouTubeのみで公開されていた「水平線」を2021年のインターハイ開会式である8月13日に配信リリース。
9月には、「ABEMA」オリジナル恋愛番組『虹とオオカミには騙されない』の主題歌となった、21st single『黄色』をリリース。
2022年8月、映画「アキラとあきら」主題歌『ベルベットの詩』を配信リリース。10月、NHK連続テレビ小説「舞いあがれ！」主題歌『アイラブユー』を配信リリース。

2023年1月17日に約4年ぶりとなる7th album『ユーモア』をリリース。
3月より開始したback number史上初の札幌・東京・愛知・大阪・福岡の5大ドームツアー“in your humor tour 2023”を即日完売で大成功させる。
8月、「怪獣のサイズ」を配信リリース。10月にはLive Blu-ray&DVD「in your humor tour 2023 at 東京ドーム」をリリースした。

2024年1月に配信シングル「冬と春」、7月にフジテレビ月9ドラマ「海のはじまり」の主題歌「新しい恋人達に」を配信リリース（9月に22nd singleとしてCDをリリース）した。
8月からは16組のゲストアーティストを迎えた初のアリーナ対バンツアー"anti sleeps tour 2024"を全国15ヶ所32公演で開催。

2025年4月カンテレ・フジテレビ系月10ドラマ「あなたが私を奪った日から」主題歌「ブルーアンバー」をリリース。 6月、モスバーガー新ブランドCM『食べるHAPPY』篇のCMソング「ある未来より愛を込めて」をリリース。さらに8月1日公開となる劇場版『TOKYO MER～走る緊急救命室～南海ミッション』の主題歌を書き下ろしの「幕が上がる」で担当した。
12月にはNHKウインタースポーツテーマソング「どうしてもどうしても」を配信リリース。

2026年5月より、back number史上最大規模となる初の5大スタジアムツアー“Grateful Yesterdays Tour 2026”を全国5カ所9公演で開催。さらに追加公演として、初のアジアツアーを台北・ソウル・香港で開催することが決定した。