Billie Paul Piper (born Leian Paul Piper; 22 September 1982) is an English actress and former singer. She initially gained recognition as a singer after releasing her debut single "Because We Want To" at age 15, which made her the youngest female singer to enter the UK Singles Chart at number one. Her follow-up single "Girlfriend" also entered at number one. In 1998, Piper released her debut studio album, Honey to the B, which was certified platinum by the British Phonographic Industry (BPI). Her second studio album, Walk of Life, was released in 2000 and spawned her third number-one single, "Day & Night". In 2003, Piper announced that she had ended her music career to focus on acting.

Piper appeared in the BBC One sci-fi series Doctor Who as Rose Tyler, companion to the Doctor, as a regular between 2005 and 2006, and additionally in 2008, 2010, and 2013. She starred as Belle de Jour in the drama series Secret Diary of a Call Girl (2007–2011), as Brona Croft/Lily in the Showtime horror series Penny Dreadful (2014–2016), and as Karen Mars in Netflix Original series Collateral (2018), for which she was nominated for a British Academy Television Award for Best Supporting Actress. Piper co-created and starred in the Sky Atlantic series I Hate Suzie (2020–2022), for which she earned a BAFTA nomination for Best Actress in 2021 and 2023.

Piper has starred in five plays since 2007 and won the 2017 Laurence Olivier Award for Best Actress in a Leading Role in a Play for her performance in Yerma, described as a "generation's best". She went on to win a total of six Best Actress awards for that performance, including the Olivier Award, making Piper the only actor to have won six out of an available six Best Actress awards for a single performance.