The Diary of Alicia Keys is the second studio album by American singer-songwriter Alicia Keys. It was released on November 30, 2003, by J Records. Almost entirely written and produced by Keys, the album is a concept album functioning as her auditory diary, primarily dealing with relationship complexities. Musically, it is predominantly a soul, R&B, contemporary classical, and hip-hop record.

While touring in support of her debut studio album Songs in A Minor (2001), Keys began writing songs for The Diary of Alicia Keys, conceiving it as an intimate record devoid of featured artists. Recording sessions commenced following the Songs in A Minor Tour's culmination in December 2002, and continued even into the international promotional tour ahead of the album's release in November 2003. Keys' then-partner and frequent collaborator Kerry Brothers Jr. largely contributed to the album, while other contributors included Timbaland, Easy Mo Bee, Kanye West, and Dre & Vidal, with whom Keys collaborated for the first time. Furthermore, Keys eschewed the neo soul style of Songs in A Minor in favor of 1960s–1970s-influenced traditional soul for The Diary of Alicia Keys.

The Diary of Alicia Keys received widespread acclaim from music critics, who generally agreed that it refuted expectations of a sophomore slump, while praising Keys' artistic maturity and vocal performance. A substantial commercial success, it debuted atop the US Billboard 200, with first-week sales of 618,000 units, and became one of the best-selling albums of 2003 and 2004, both nationally and globally. The album yielded four singles, three of which—"You Don't Know My Name", "If I Ain't Got You", and "Diary"—reached the top 10 on the US Billboard Hot 100 and were among biggest hits of 2004 in the US. To further promote The Diary of Alicia Keys, Keys embarked on the Verizon Ladies First Tour (2004), co-headlined with Beyoncé and Missy Elliott, and the Diary Tour (2004–2005).

The Diary of Alicia Keys earned Keys nominations for numerous industry awards, and won her her second Grammy Award for Best R&B Album, among other awards. In retrospective commentaries, a multitude of critics stated that the album solidified Keys' status in the music industry, and extended her critical and commercial success past her debut. By the release of its successor As I Am (2007), The Diary of Alicia Keys had sold over eight million copies worldwide. In 2020, it was certified quintuple platinum by the Recording Industry Association of America (RIAA), for combined sales and album-equivalent units of five million in the US. In commemoration of its 20th anniversary, the album was reissued as The Diary of Alicia Keys 20 in December 2023, and won a Grammy Award for Best Immersive Audio Album.

Alicia Keys' debut studio album Songs in A Minor was released on June 26, 2001, by J Records. A fusion of classical music, traditional and modern styles of soul, and contemporary R&B, it received widespread critical acclaim, being hailed as an auspicious and accomplished debut. Preceded by the US Billboard Hot 100 number-one single "Fallin'", the album debuted atop the US Billboard 200 with first-week sales of 236,000 units. It earned Keys five Grammy Awards at the 44th Annual Grammy Awards (2002), which tied Keys with Lauryn Hill as the female artist with most Grammy Awards won in a single ceremony. The album is widely regarded as an immensely influential record of its era, and is credited with propelling Keys to global prominence, placing her at the forefront of neo soul. One of the best-selling albums of 2001 and 2002, Songs in A Minor had sold over 10 million copies worldwide by November 2003. Consequently, substantial pressure was placed on The Diary of Alicia Keys to match or exceed that success, as to avoid the sophomore slump. Keys, however, felt confident due to her experience from the creation of Songs in A Minor. Keys' then-partner and collaborator Kerry Brothers Jr. would retrospectively state that Keys having creative control over her artistry since her debut helped them overcome the concerns, as they strived to replicate, but enhance, the formula of Songs in A Minor. While on the Songs in A Minor Tour (2002), Keys began developing ideas for what would become The Diary of Alicia Keys. Conceiving it as an intimate record, Keys refrained from seeking collaborators out to contribute to the album, despite distinguished collaborations with Eve and Christina Aguilera in 2002.

Keys began writing songs for what would become The Diary of Alicia Keys as early as 2001, with "If I Ain't Got You" being among the first songs written. She felt inspired to write it while on a flight, after hearing the news of Aaliyah's death in a plane crash in August 2001, as flying inside a plane against the circumstances of Aaliyah's demise presented her with a "sentiment of being present in the moment". As Christina Aguilera asked Keys to write a song for her fourth studio album Stripped (2002), Keys considered proffering "If I Ain't Got You" to her, but was persuaded otherwise by J Records' then-executive vice president of artists and repertoire (A&R) Peter Edge; Keys ultimately wrote "Impossible" for Aguilera. The songwriting for The Diary of Alicia Keys became continual during the Songs in A Minor Tour (2002), with Keys writing "Dragon Days" and "Feeling U, Feeling Me", amongst other songs, while touring. Following the tour's completion in December 2002, she began recording the album with audio engineers Ann Mincieli and Tony Black. Like Songs in A Minor, Keys executive produced The Diary of Alicia Keys, alongside Edge and her then-manager Jeff Robinson, and primarily collaborated with Kerry Brothers Jr., her former EmBishion fellow member Taneisha Smith, and songwriter Erika Rose. Brothers and Smith wrote "Karma" together while Keys was recording another track; Brothers originally produced it for an undisclosed rapper in 2000, making it the album's sole track not produced by Keys. Alongside "Karma", Brothers also co-wrote "Diary", "Wake Up", and "When You Really Love Someone". Smith co-wrote "Nobody Not Really" and eventual bonus track "Streets of New York (City Life)", while Rose co-wrote "Heartburn", "Slow Down", and "Samsonite Man".

In order to capture the desired 1960s–1970s sound on The Diary of Alicia Keys, Keys equipped the studios at which she recorded with live instruments. Initial recording sessions took place at the Kampo Studio in New York City, as Keys refrained from recording at locations frequently used by her contemporaries in order to concentrate solely on her work. "Samsonite Man" and "Nobody Not Really", which would ultimately become the album's closing tracks, were the first tracks recorded, and were used as guidelines for the remainder of the production. At Kampo, Keys also recorded the Easy Mo Bee-produced "If I Was Your Woman"/"Walk on By", a vocal cover of "If I Were Your Woman" (1970) by Gladys Knight & the Pips against an instrumental sample from Isaac Hayes' 1964 song "Walk On By". As it is a cover, it is the album's sole track not written by Keys. Keys originally recorded a standalone alternative rock-styled cover of "If I Was Your Woman" for Songs in A Minor. After the track was left unused, Keys re-recorded it against a sample from "Walk On By", inspired by Easy Mo Bee's previous production of The Notorious B.I.G.'s 1994 track "Warning", also using a sample from "Walk On By". The original demo of "If I Was Your Woman" would be released via 10th anniversary editions of Songs in A Minor in 2011. Both versions were co-produced by D'Wayne Wiggins. Besides Kampo, New York recording sessions for The Diary of Alicia Keys materialized at Keys' own KrucialKeys Studios, co-founded with Brothers, as well as The Hit Factory and the Quad Recording Studios.

Apart from Easy Mo Bee, new collaborators on The Diary of Alicia Keys included Timbaland, Kanye West, and Dre & Vidal. Timbaland co-wrote and produced "Heartburn", the only track recorded at Hit Factory Criteria in Miami, while West composed and produced "You Don't Know My Name". Harold Lilly, who co-wrote it, and then-obscure John Legend contributed background vocals to the latter. Keys fortuitously met Dre & Vidal as they were working on Usher's Confessions (2004) at the same studio as her. They ultimately collaborated during a three-day session at Quad, writing and producing "So Simple" together. For "So Simple", Keys introduced her alter ego Lellow, which Keys' collaborators would refer to her as whenever she recorded influenced by hip-hop, as a "featured artist". After the sessions at Quad, the track was finished at The Crib in Philadelphia. Recording and production of tracks such as "You Don't Know My Name" and "If I Ain't Got You", at The Hit Factory, was interrupted by the August 14–16, 2003 Northeast blackout. By the time The Diary of Alicia Keys reached its completion in November 2003, Keys had already embarked on the album's promotional tour across Europe. "When You Really Love Someone", "Feeling U, Feeling Me", and "Slow Down" were partly or fully recorded at Sarm West in London. "Diary", for which Stokley Williams of Mint Condition originally provided additional vocals, was finished at the Soundwise Studio in Amsterdam, after Jermaine Paul, who joined Keys as a background vocalist for the promotional tour, was deemed better suited for the song. "Harlem's Nocturne", the final track recorded for the album, was recorded at Plus XXX in Paris and finished at The Hit Factory in New York, where The Diary of Alicia Keys was mastered by Herb Powers, Jr.