John Warren Geils Jr. (/ɡaɪlz/) (February 20, 1946 – April 11, 2017), known professionally as J. Geils or Jay Geils, was an American guitarist. He was known as the leader of The J. Geils Band.

Growing up in New York City, Geils became interested in jazz and blues. After moving to Massachusetts for his college education, he formed the J. Geils Blues Band while still a student at Worcester Polytechnic Institute. After dropping the word “Blues” from their name, the band released their first album in 1970, performing soul and rhythm and blues-influenced rock music for most of the 1970s before turning to pop music in the 1980s. After the band broke up in 1985, Geils left regular performing to take up restoration and racing of automobiles, with occasional forays into music production. He continued to appear in reunion tours with the rest of his band sporadically during the 2000s and 2010s.

John Warren Geils Jr. was born on February 20, 1946, in New York City, and grew up in Morris Plains, New Jersey. He was of German ancestry.