Jack Halberstam (born December 15, 1961), also known as Judith Halberstam, is Professor of American Studies and Ethnicity, Gender Studies, and Comparative Literature, as well as serving as the Director of The Center for Feminist Research at University of Southern California (USC). Halberstam was the Associate Professor in the Department of Literature at the University of California at San Diego before working at USC. He is a gender and queer theorist and author.

Halberstam, who accepts masculine and feminine pronouns, as well as the name "Judith," with regard to his gender identity, focuses on the topic of tomboys and female masculinity for his writings. His 1998 Female Masculinity book discusses a common by-product of gender binarism, termed "the bathroom problem" with outlining the dangerous and awkward dilemma of a perceived gender deviant's justification of presence in a gender-policed zone, such as a public bathroom, and the identity implications of "passing" therein.

Jack is a popular speaker and gives lectures in the United States and internationally on queer failure, sex and media, subcultures, visual culture, gender variance, popular film and animation. Halberstam is currently working on several projects including a book on fascism and (homo)sexuality.