Alexandra Byrne (born 1962) is an English costume designer. Much of her career has focused on creating costumes for period dramas. These films include Persuasion (1995), Hamlet (1996), Elizabeth (1998), Finding Neverland (2004), The Phantom of the Opera (2004), Elizabeth: The Golden Age (2007), Mary Queen of Scots (2018), The Aeronauts (2019), and Emma. (2020). She has been nominated for the Academy Award for Best Costume Design six times, winning once for Elizabeth: The Golden Age.

Since 2011, Byrne has also designed the costumes for many films in the Marvel Cinematic Universe, including Thor (2011), The Avengers (2012), Guardians of the Galaxy (2014), Avengers: Age of Ultron (2015), Doctor Strange (2016), and The Fantastic Four: First Steps (2025).

Byrne was born in Hampshire and grew up amid many artistic influences. She was raised in Stratford-upon-Avon, where the Royal Shakespeare Company is based. She would later tell an interviewer that her career in theatre design was predestined, and she began by studying architecture with the intention of building sets in theatre. She encountered costume design when studying theatre design with the English National Opera. She later said, "I worked in the theatre for quite a few years, doing costume and set design. In England, it's very much both." She also trained with the Motley Theatre Design Course.

Byrne oversaw scenery and costumes for the 1989 comedy play Some Americans Abroad. Her work earned her a Tony Award nomination for Best Scenic Design. Despite her success, she was unsure if she would focus on set design or costume design. She decided on the latter when she was working on a television project for the BBC's Shakespeare series. Byrne said, "I was assisting a costume designer who was fantastic with fabric. It was with her that I actually thought, 'This is what it's all about. This is what I want to do.".

In 1993, Byrne created the costumes for all four parts of the TV serial The Buddha of Suburbia, starring Naveen Andrews. For her costume work in the production, she received a nomination at the British Academy Television Awards.

Byrne's first encounter with period costume dramas began when she was hired to create the costumes for the BBC film Persuasion. The production was an adaptation of the novel of the same name by Jane Austen and featured Amanda Root as Anne Elliot. Byrne had also worked with the film's director, Roger Michell, on Some Americans Abroad and The Buddha of Suburbia. For her work in Persuasion, Byrne won the British Academy Television Award for Costume Design. The following year, Byrne created the costumes for the 1996 film Hamlet directed by and starring Kenneth Branagh, with whom she had previously worked on Life of Napoleon. She received her first Academy Award nomination for Best Costume Design.

Byrne married English actor Simon Shepherd in 1980.

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