William Thomas Hader Jr. (born June 7, 1978) is an American actor, comedian, director, producer and writer. He gained widespread attention for his eight-year stint as a cast member on the long-running NBC sketch comedy series Saturday Night Live from 2005 to 2013, for which he received four Primetime Emmy Award nominations and a Peabody Award. He became known for his impressions and especially for his work on the Weekend Update segments, where he played Stefon Meyers, a flamboyant New York City nightclub tour guide.

Hader co-created the HBO dark comedy series Barry (2018–2023) with Alec Berg, in addition to playing the title role as Barry Berkman. He also served as producer, writer and director, for which his efforts garnered him eight Emmy Award nominations for the series. He won two, consecutively, for Outstanding Lead Actor in a Comedy Series. He is a star and producer of the IFC mockumentary comedy series Documentary Now! (2015–present) along with Fred Armisen and Seth Meyers. He was Emmy-nominated for his guest role in Curb Your Enthusiasm in 2022.

Hader has had supporting roles in the films Hot Rod (2007), Superbad (2007), Forgetting Sarah Marshall (2008), Adventureland (2009) and The BFG (2016), and leading roles in The Skeleton Twins (2014), Trainwreck (2015), and as an adult Richie Tozier in It Chapter Two (2019). He has also done extensive voice work, portraying leading and supporting characters in films such as the Cloudy with a Chance of Meatballs franchise (2009–2013), Turbo (2013), the Oscar-winning original Inside Out (2015), The Angry Birds Movie (2016) and its 2019 sequel, Power Rangers (2017) and Toy Story 4 (2019).