Lucy Boynton (born 17 January 1994) is a British actress. Born in New York City and raised in London, she made her professional debut as the young Beatrix Potter in Miss Potter (2006).

She appeared in television productions Ballet Shoes (2007), Sense and Sensibility (2008) and Mo (2010), making guest appearances on Lewis, Borgia, Endeavour, and Law & Order: UK. Boynton portrayed writer Angelica Garnett on Life in Squares, which aired on BBC. She appeared as an isolated popular girl in The Blackcoat's Daughter (2015) and starred as a bold aspiring model in Sing Street (2016). She also appeared in horror films I Am the Pretty Thing That Lives in the House (2016) and Don't Knock Twice (2016).

Boynton played Countess Andrenyi in Murder on the Orient Express (2017), which was a commercial success. Boynton portrayed an addict in Netflix's Gypsy (2017) and the daughter of a cult leader in Apostle (2018). Boynton gained recognition for starring as Mary Austin in the biopic Bohemian Rhapsody (2018). She appeared in HBO Max's Locked Down (2021). Boynton appeared as the privileged antagonist Astrid Sloan in the Netflix series The Politician, appeared in the second season of Modern Love, and starred in ITV's The Ipcress File (2022).

Boynton was born in New York to English parents, journalists Graham Boynton and Adrianne Pielou. The younger of two daughters, Boynton was initially raised in New York until the age of four when her family returned to Britain; thereafter, she was raised in southeast London. She retains dual citizenship of the United Kingdom and the United States. Boynton realized she wanted to act when she was ten, after a drama teacher taught her that "acting was not playing pretend; it was understanding the human mind and why people function as they do." She attended the private Blackheath High School, followed by James Allen's Girls' School in Dulwich.

Boynton made her professional debut at age twelve as young Beatrix Potter in the biographical film Miss Potter (2006). She was asked to audition after being spotted by a casting director sitting in on her drama class. Additional scenes were written for her character after a test screening responded positively to her role. Boynton wore corsets and padded clothing to portray a younger Potter. Boynton stated that the first day of filming was "the best day of [her] life". In 2007, she was nominated for the Young Artist Award for Best Supporting Actress, for Miss Potter. Boynton starred as Posy Fossil in the BBC film Ballet Shoes (2007). A body double was used in some scenes to display her character's "remarkable dancing skill". The film was praised by critics.

Boynton portrayed Margaret Dashwood in the 2008 television adaptation of Jane Austen's novel Sense and Sensibility. The miniseries premiered to high viewership and positive reviews. Boynton has described experiencing a "difficult period" from ages 16 to 17 for being "too old for young roles, but too young to play the leading lady." She played Mo Mowlam's stepdaughter in television film Mo (2010). In 2011, She appeared in an episode of the ITV drama Lewis. In 2013, she starred in Saint Raymond's music video for the lead single from his debut extended play, "Fall At Your Feet". Boynton appeared in period drama Copperhead (2013) based on the novel of the same name by Harold Frederic. In 2014, she guest-starred in the television series Borgia, Endeavour, and Law & Order: UK. She played Angelica Garnett, a member of the Bloomsbury Group, in the BBC miniseries Life in Squares, which aired in 2015. To prepare for the part, she read Garnett's memoir, Deceived with Kindness, in which most of her scenes were detailed, which she described as "a dream for any actor". The series was reviewed positively.

Boynton appeared as "sullen mean girl" Rose alongside Emma Roberts and Kiernan Shipka in The Blackcoat's Daughter (2015). She prepared for the film by watching films that dealt with isolation and grief, such as Rosemary’s Baby on the recommendation of director Oz Perkins. Chuck Bowen of Slant Magazine wrote that Roberts, Shipka, and Boyton were "poignant in their minimalist roles". Boynton starred in the coming-of-age film Sing Street (2016) as Raphina, an aspiring model with "huge hair, bright make-up and colourful clothes". Boynton described her role as "the muse character, in a way, but only because she has put herself there". She adopted an Irish accent for the film and was instructed by director John Carney to create a backstory "in much more detail than [she] had before." The film premiered to critical praise at the Sundance Film Festival and garnered a Golden Globe nomination for Best Motion Picture – Musical or Comedy.

She appeared as ghostly bride Polly Parsons in the gothic-horror film I Am the Pretty Thing That Lives in the House (2016), which opened at the Toronto International Film Festival to mixed reception. Boynton starred alongside Katee Sackhoff in indie-horror Don't Knock Twice (2016). Noel Murray of the Los Angeles Times praised Sackhoff and Boynton's "volatile chemistry", stating that "they bring so much life to the material that it’s almost like they’ve been tricked into thinking they’re in a better movie."