Denis E. Dillon (December 21, 1933 – August 15, 2010), was an American prosecutor and politician who served as District Attorney of Nassau County, New York, from 1975 to 2005. Dillon was well known for his opposition to abortion rights, and the issue prompted his defection to the Republican Party in 1989, having previously been one of the very few Democratic politicians to have success in Nassau County. Prior to his defection, Dillon challenged incumbent Democrat Mario Cuomo from the right in the 1986 New York gubernatorial election, finishing in third place with 3% of the vote as the nominee of the New York State Right to Life Party.