William Jack Poulter (born January 28, 1993) is an English actor. He first gained recognition for his role as Eustace Scrubb in the fantasy adventure film The Chronicles of Narnia: The Voyage of the Dawn Treader (2010). He received critical praise for his starring role in the comedy film We're the Millers (2013), for which he won the BAFTA Rising Star Award.

Poulter starred in the dystopian science fiction film The Maze Runner (2014) and the sequel Maze Runner: The Death Cure (2018), the period epic film The Revenant (2015), the crime drama film Detroit (2017), the interactive science fiction film Black Mirror: Bandersnatch (2018), and the folk horror film Midsommar (2019). In 2021, he had a leading role in the Hulu miniseries Dopesick, for which he received an Emmy nomination for Outstanding Supporting Actor in a Limited or Anthology Series or Movie. In 2023, he joined the Marvel Cinematic Universe as Adam Warlock in Guardians of the Galaxy Vol. 3.