William Thomas "Bill" Hader Jr. (born June 7, 1978 in Tulsa, Oklahoma) is an American comedian, actor, voice actor, producer, and writer, who is best known for being a cast member on the NBC late-night sketch comedy and variety show Saturday Night Live from 2005 to 2013. He is also known for his lead voice role as Flint Lockwood in Cloudy with a Chance of Meatballs and its 2013 sequel, and for his supporting roles in various comedy films, such as Superbad, Night at the Museum: Battle of the Smithsonian, Power Rangers, Pineapple Express, Forgetting Sarah Marshall, Tropic Thunder, and Adventureland. He is currently a writer and occasional voice actor for the animated series South Park.

He voiced the referee and Slug Monster Student in the 2013 Pixar film Monsters University, Fear in the 2015 Pixar film Inside Out, Leonard in the 2016 computer-animated comedy film The Angry Birds Movie and its 2019 sequel, Stan in the 2016 Pixar film Finding Dory, and portrayed Bloodbottler in the 2016 fantasy film The BFG. He will play Santa Claus' son in Noelle.