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.