Joseph Donald Mascis Jr., better known as J Mascis, is an American musician who is the singer, guitarist and main songwriter for the alternative rock band Dinosaur Jr. He has also released several albums as a solo artist and played drums and guitar on other projects. He was ranked number 74 in a Rolling Stone list of the "100 Greatest Guitarists", and number 5 in a similar list for Spin magazine in 2012.

Dinosaur Jr. is an American rock band formed in Amherst, Massachusetts, in 1984. Originally called Dinosaur, the band was forced to change their name due to legal issues.