Harriet Marston Slater (born 28 November 1994) is an English actress. On television, she is known for her roles in the DC Universe series Pennyworth (2019–2022) and the MGM+ period drama Belgravia: The Next Chapter (2024). She was named a 2023 Bright Young Thing by Tatler

Slater was born and raised in Leicester. Her extended family were active in amateur dramatics. She joined a local youth theatre group called the Little Theatre at the age of 6. She found acting a stress-reliever from anxiety. Slater has mentioned Lindsay Lohan and Jane Fonda as influences when young. She graduated with a Bachelor of Arts in Acting from the Guildford School of Acting in 2016.

After graduating from drama school, Slater immediately began her career in regional theatre, making her professional debut in The Man with the Hammer at the Theatre Royal, Plymouth in 2016. This was followed by a role in the 2017 Royal Shakespeare Company production of Phil Porter's Vice Versa at the Swan Theatre. She also appeared in The Secret Seven at Storyhouse.

Slater made her television debut when she was cast as Sandra Onslow in the DC Universe series and Batman prequel Pennyworth, which premiered on Epix in 2019. After playing a recurring character during the first season, she joined the main cast for the subsequent two seasons. She made her feature film debut in the 2020 indie fantasy film Emily and the Magical Journey. In 2023, she had a small role in the film Indiana Jones and the Dial of Destiny.

In 2024, Slater starred as Clara Trenchard (née Dunn) in Helen Edmundson's period drama Belgravia: The Next Chapter for MGM+. In February 2024, it was announced that she had been cast as Ellen MacKenzie in the Outlander prequel series Outlander: Blood of My Blood. She appeared in the films Tarot (also known as Horrorscope) and True Haunting.