The GI Underground Press was an underground press movement that emerged among the United States military during the Vietnam War. These were newspapers and newsletters produced without official military approval or acceptance; often furtively distributed under the eyes of "the brass". They were overwhelmingly antiwar and most were anti-military, which tended to infuriate the military command and often resulted in swift retaliation and punishment. Mainly written by rank-and-file active duty or recently discharged GIs, AWOLs and deserters, these publications were intended for their peers and spoke the language and aired the complaints of their audience. They became an integral and powerful element of the larger antiwar, radical and revolutionary movements during those years. This is a history largely ignored and even hidden in the retelling of the U.S. military's role in the Vietnam War.

The terms underground press or clandestine press refer to periodicals and publications that are produced without official approval, illegally or against the wishes of a dominant group. In specific recent Asian, American and Western European context, the term "underground press" has most frequently been employed to refer to the independently published and distributed underground papers associated with the counterculture of the late 1960s and early 1970s in India and Bangladesh in Asia, in the United States and Canada in North America, and the United Kingdom and other western nations. It can also refer to the newspapers produced independently in repressive regimes. In German occupied Europe, for example, a thriving underground press operated, usually in association with the Resistance. Other notable examples include the samizdat and bibuła, which operated in the Soviet Union and Poland respectively, during the Cold War.