21 is the second studio album by English singer-songwriter Adele. It was released on 24 January 2011 in Europe by XL Recordings and on 22 February 2011 in North America by Columbia Records. The album was named after the age of the singer during its production. 21 shares the Motown/soul influences of her 2008 debut album 19, but also draws influence from the American country and Southern blues music that Adele started listening to during the North American leg of her tour An Evening with Adele. Composed in the aftermath of the singer's separation from her then partner, the album explores themes of heartbreak, healing, introspection, and forgiveness.

Adele began writing 21 in April 2009, when still involved in the relationship that subsequently inspired the record. Looking to deviate from the brooding sound of her first album, she had intended to compose a more upbeat and contemporary follow-up. However, studio sessions ended prematurely due to a lack of inspiration. She resumed production immediately after the breakdown of her relationship, channelling her heartbreak and depression into her songs. Adele collaborated with various songwriters and producers, including Columbia Records co-president at the time Rick Rubin, Paul Epworth, Ryan Tedder, Jim Abbiss, and Dan Wilson.

A sleeper hit, 21 defied the modest commercial expectations of Adele's independent record label XL. It topped record charts in more than 30 countries and became the world's best-selling album of the year for both 2011 and 2012, helping to revitalise the lagging sales of the global music industry. In the United Kingdom, it is the best-selling album of the 21st century, the fourth best-selling album of all time, and the best-selling album by a solo artist of all time, while its 23-week tenure atop the UK Albums Chart is the longest by a female solo artist. In the United States, it is the best performing Billboard 200 album of all time, holding the top position for 24 weeks, longer than any other album since 1985 and the longest by a female solo artist in Billboard 200 history. It was certified Diamond by the Recording Industry Association of America (RIAA), denoting sales of over 10 million copies in the US. In addition, three of the five singles released in its promotion – "Rolling in the Deep", "Someone like You" and "Set Fire to the Rain" – became international number-one songs, while "Rumour Has It" charted in the top 20 in countries across Europe and North America. With sales of over 31 million copies worldwide, 21 is the best-selling album of the 21st century, and one of the best-selling albums of all time.

Praised for its understated production, vintage aesthetic, and Adele's vocal performance, 21 was shortlisted for the 2011 Mercury Prize, and won the 2012 Grammy Award for Album of the Year and the Brit Award for British Album of the Year. It has since been ranked amongst the Rolling Stone's 500 Greatest Albums of All Time, and is included in the book 1001 Albums You Must Hear Before You Die.