William Emerson Arnett (/ɑːrˈnɛt/; born May 4, 1970) is a Canadian-American actor and comedian. He is widely known for his roles as Gob Bluth in the Fox/Netflix sitcom Arrested Development (2003–2006, 2013, 2018–2019) and the title character in the Netflix animated series BoJack Horseman (2014–2020). He has received nominations for seven Primetime Emmy Awards and three Screen Actors Guild Awards.

Arnett was nominated for an Emmy for his recurring role as Devon Banks in the NBC sitcom 30 Rock (2007–2013). He appeared in the NBC sitcom Up All Night (2011–2012), the CBS sitcom The Millers (2013–2014), and the Netflix detective series Murderville. He also starred in, co-wrote, and executive produced the Fox sitcom Running Wilde (2010–2011) and the Netflix comedy-drama series Flaked (2016–2017).

Arnett has appeared in such films as Let's Go to Prison (2006), Blades of Glory (2007), Semi-Pro (2008), G-Force (2009), and Teenage Mutant Ninja Turtles (2014). Known for his deep baritone voice, Arnett has also voiced roles in Danny Phantom (2004–2007), Ice Age: The Meltdown (2006), Ratatouille (2007), Horton Hears a Who! (2008), Monsters vs. Aliens (2009), Despicable Me (2010), the Die Hard franchise (2014–2017), The Lego Movie franchise (2014–2019), Teen Titans Go! (2018), Chip 'n Dale: Rescue Rangers (2022), and Twisted Metal (2023–).

Arnett has been nominated for a Primetime Emmy Award for Arrested Development, 30 Rock, and BoJack Horseman. Since 2020, he has hosted the Fox reality show Lego Masters. Arnett began co-hosting the comedy podcast SmartLess (2020–) with Sean Hayes and Jason Bateman.