Peter John Sinfield (born 27 December 1943) is an English poet and songwriter. He is best known as a co-founder and former lyricist of King Crimson. Their debut album In the Court of the Crimson King is considered one of the first and most influential progressive rock albums ever released.

Sinfield's lyrics are known for their surreal imagery, often involving common fantasy concepts, nature, or the sea. They often also deal with emotional concepts and, sometimes, storyline concepts. Later in his career, he adapted his songwriting to better suit pop music, and wrote a number of successful songs for artists such as Celine Dion, Cher, Cliff Richard, Leo Sayer, Five Star, and Bucks Fizz.

Paul Stump, in his 1997 History of Progressive Rock, called Sinfield "the premier littérateur of Progressive". In 2005, Sinfield was referred to as a "prog rock hero" in Q magazine for his lyrical work and influence in the music industry.