J Mascis (born Joseph Donald Mascis) is an American musician, best known as the singer, guitarist and songwriter for Dinosaur Jr.. In 2011, he was ranked 86th in Rolling Stone's list of the 100 greatest guitarists of all time.

In the early 1980s he founded the short-lived hardcore group Deep Wound with his high school friends Lou Barlow and Scott Helland. He started out on drums, but moved to guitar when he went on to found Dinosaur Jr. (with bassist Barlow and drummer Patrick “Murph” Murphy in 1984), which achieved national and international success. After Mascis dismissed Barlow from Dinosaur Jr. in 1989, he recorded several more Dinosaur Jr. albums until 1997, including the 1996 acoustic solo album Martin & Me. He then went on to produce albums with his new band, J Mascis and the Fog, starting in 2000.

In April 2005, Mascis joined up again with Barlow and Murph, as the original trio of Dinosaur Jr., to begin a tour celebrating the re-release of the group’s first three albums.

In August 2005, Mascis released a new solo album as J Mascis and Friends, J and Friends Sing and Chant for Amma, an album of devotional songs dedicated to Hindu religious leader Mata Amritanandamayi, or Ammachi, about whom he had previously written “Ammaring” on the first J Mascis and the Fog album More Light. The proceeds from the album are being donated to Tsunami relief efforts by Ammachi’s organisation.

In 2006, Mascis returned to drumming with his newly formed heavy metal band Witch for their self-titled debut album.

In 2010, Mascis joined with John Petkovic and Tim Parnin of Cobra Verde and Dave Sweetapple of Witch to form Sweet Apple. The self-titled debut album was released on Tee Pee Records Mascis plays drums, guitar, and sings on the album.

Mascis released a mostly acoustic album in March, 2011 titled Several Shades Of Why on Sub Pop Records. He was joined in the studio by several guest musicians including Kurt Vile and Sophie Trudeau