Marvin John "Jack" Nance (December 21, 1943 – December 30, 1996) was an American actor. A frequent collaborator of filmmaker David Lynch, he starred in Lynch's directorial debut Eraserhead (1977) and continued to work with Lynch throughout his career, including a recurring role as Pete Martell on Twin Peaks (1990–1991).

Marvin John Nance was born in Boston, Massachusetts, on December 21, 1943, the son of Agnes (née O'Grady) and Neiman Marcus executive Marvin Hoyt Nance. He grew up in the Oak Cliff neighborhood of Dallas, Texas. During childhood Nance was struck by a car, which injured his back. He graduated from South Oak Cliff High School and studied journalism at North Texas State University where he began acting. Nance dropped out of school to pursue acting full-time and joined the Dallas Theater Center.

In 1964, Nance headed for California and worked for some time with the American Conservatory Theater in San Francisco. He had success and landed the lead role in a play based on the life of Thomas Paine, which was directed by David Lindemann. He was considered for the role of Benjamin in The Graduate (1967), which went to Dustin Hoffman, and for the role of Perry Smith in In Cold Blood (1967), which went to Robert Blake. He played twin brothers Benny and Tony Rebozo in the Doo Dah Gang, a performance group that staged 1920s-style gang fights at nontraditional venues. When one of his characters died, he spent three days lying in a coffin at the staged wake.

Nance met filmmaker David Lynch in 1972 after an introduction by Lindemann. Lynch was a fellow at the American Film Institute and was creating a 20-minute short film in Beverly Hills with $10,000. Lindemann recommended Nance for the role of Henry Spencer in Eraserhead (1977), which was filmed sporadically over five years due to Lynch's funding problems. Eraserhead was initially unsuccessful but slowly became a midnight movie, leading to its cult classic status. Nance's performance has since been praised. The New York Times wrote in 1980 that his "minimalist features, unchanging expression, tight dark suit, and short, almost crippled steps suggest many silent-film comedians". The film became a favorite of Stanley Kubrick, who required the cast of The Shining to watch it before filming began.

Nance continued to work with Lynch in subsequent projects during his lifetime, appearing in Dune (1984), Blue Velvet (1986), The Cowboy and the Frenchman (1988), Wild at Heart (1990), Twin Peaks (1990–1991), Twin Peaks: Fire Walk with Me (1992), and Lost Highway (1997). While none of the scenes he filmed for Fire Walk with Me appeared in the theatrical cut, they were later compiled in Twin Peaks: The Missing Pieces (2014).

Nance appeared with actress Mary Woronov as a married couple in the music video for the 1983 Suicidal Tendencies song "Institutionalized". After gaining sobriety in 1986, and needing a steady salary, Nance took courses in hotel management. He began working as a clerk at Hotel Hollywood. While there, he refused a role in Miracle Mile (1988) due to it conflicting with his job schedule. Dennis Hopper hired him to appear in Colors (1988) after they had both appeared in Blue Velvet. He also had a brief appearance in The Blob (1988).

In 1990, Nance was offered the role of Pete Martell in Lynch's series Twin Peaks, appearing throughout the show's original run. After an addiction relapse in 1993, his life took another turn and he had smaller roles in films that were not as successful. He lived in a hotel, from which he was kicked out for firing a gun at his television. He guest-starred on a 1995 episode of My So-Called Life called "Weekend", in which he played an innkeeper, and continued in smaller roles until his death.

In 2017, 21 years after his death, Nance appeared in the third season of Twin Peaks via footage of his character Pete Martell from the pilot episode of the original series. The episode "Part 17" was dedicated to his memory. His final performance, in Michael Moriarty's Of Things Past, was filmed in 1985 but not released until 2023.