Louis Alfred Székely (/ˈluːi ˌseɪˈkeɪ/; born September 12, 1967), known professionally as Louis C.K. (/ˈluːi ˌsiːˈkeɪ/), is an American stand-up comedian, actor and filmmaker. C.K. has won six Emmy Awards, and three Grammy Awards, three Critics' Choice Awards, three Peabody Awards and a Screen Actors Guild Award as well as nominations for two Golden Globe Awards. He was listed as one of Time Magazine's 100 Most Influential People in 2012 and Rolling Stone ranked him fourth on its 2017 list of the 50 best stand-up comics of all time.

C.K. began his career in the 1990s writing for comedians including David Letterman, Conan O'Brien, Dana Carvey, Chris Rock, and Saturday Night Live. He was also directing surreal short films and directed two features—Tomorrow Night (1998) and Pootie Tang (2001). In 2001, C.K. released his debut comedy album, Live in Houston, directly through his website and became among the first performers to offer direct-to-fan sales of tickets to his stand-up shows and DRM-free video concert downloads via his website. He became prolific releasing nine comedy albums, often directing and editing his specials as well. These specials include Shameless (2007), Chewed Up (2008), Hilarious (2010), and Oh My God (2013).

He gained prominence and widespread acclaim for his FX semi-autobiographical comedy-drama series Louie (2010–2015), which he created, directed and starred in. The series received numerous accolades with C.K. winning two Primetime Emmy Award for Outstanding Writing in a Comedy Series. In 2016, he created and starred in his self-funded web series Horace and Pete, and co-created the shows Baskets and Better Things for FX. In film, he acted in American Hustle (2013), Blue Jasmine (2013), Trumbo (2015), and The Secret Life of Pets (2016).

In 2017, he admitted to several incidents of sexual misconduct following the release of an article in The New York Times. This resulted in widespread criticism and caused his 2017 film I Love You, Daddy to be pulled from distribution prior to its release. In 2018, he returned to stand-up comedy, and in 2019, he announced an international tour. He has also released the specials Sincerely Louis C.K. (2020) and Sorry (2021) on his website, receiving a Grammy Award for Best Comedy Album for the former. C.K. also co-wrote and directed the film Fourth of July (2022).

Louis Alfred Székely was born in Washington, D.C., on September 12, 1967, the son of software engineer Mary Louise (née Davis) and economist Luis Székely. He has three sisters. His father is of Mexican and Hungarian-Jewish descent. C.K.'s Jewish paternal grandfather, Géza Székely Schweiger, had immigrated from Hungary to Mexico; he and his Mexican wife, Rosario Sánchez Morales, raised their children in the Catholic faith. C.K.'s mother, an American, was a Catholic with Irish ancestry.

When C.K. was an infant, his family moved to his father's home country of Mexico, where his father had earned a degree from the National Autonomous University of Mexico prior to graduating from Harvard. C.K.'s first language was Spanish; it was not until after they moved back to the U.S. when he was 7 that he began to learn English. He has said that he has since forgotten much of his Spanish. When C.K. left Mexico with his family, they moved back to the United States and settled in the Boston area, initially for a year in Framingham, Massachusetts.

Upon moving from Mexico to suburban Boston, C.K. wanted to become a writer and comedian, citing George Carlin, Bill Cosby and Richard Pryor as some of his influences. When he was 10, his parents divorced. C.K. said that his father was around but he did not see him much and when he remarried, C.K.'s father converted to Orthodox Judaism, the faith of his new wife. C.K. and his three sisters were raised by their single mother in Newton, Massachusetts. The fact that his mother had only "bad" TV shows to view upon returning home from work inspired him to work on television. C.K.'s mother raised her children as Catholic and they attended after-school Catholic class until they completed communion. C.K. has said that his father's whole family still lives in Mexico. C.K.'s paternal uncle Dr. Francisco Székely is an academic and an international consultant on environmental affairs who served as Mexico's Deputy Minister of Environment (2000–2003).

C.K. attended Newton North High School and graduated in 1985. He graduated with future Friends star Matt LeBlanc. After graduation, C.K. worked as an auto mechanic and at a public access TV cable station in Boston. According to C.K., working in public access TV gave him the tools and technical knowledge to make his short films and later his television shows. "Learning is my favorite thing", he said. He also worked for a time as a cook and in a video store.