The Radical Student Union (RSU) first appeared at UMass Amherst in 1976 as the Revolutionary Student Brigade (RSB). Founded by Charles Bagli, a Boston University graduate who had moved to Amherst to capitalize on the political momentum on campus, the RSB emerged out of a 1969 schism within the Students for a Democratic Society that produced The Revolutionary Student Movement 1 (later to become the Weather Underground) and The Revolutionary Student Movement 2. This latter organization, comprised of Communists and Socialists of various ideological persuasions, quickly divided and regrouped, leading to the creation of several groups including the Revolutionary Union in 1971, which set up a youth wing in 1972 called the Attica Brigade, which itself split off and formed its own organization in 1975, the Revolutionary Student Brigade.

The UMass Amherst Chapter of the RSB began slowly, with Bagli and other activists distributing material in the Campus Center during the 1975-76 school year. By the spring of 1976, the organization achieved official recognition as a student organization, with the mission of promoting the struggles and consciousness of students through the concrete application of Marxism-Leninism-Mao-Tsetung thought. Taking an office on the second floor of the Student Union building, they began to receive funding from the Student Government Association and a developed a core group of students.

The next thirty years of RSU history can be divided roughly into three phases: the founding and early development of the organization; the robust years of the mid- to late-1980s; and the slow waning of the organization since. Across this time, several principles of the group have remained constant. First, the RSU has always intended to be a multi-issue group whose focus is determined by its membership. Second, the RSU is committed to working with other groups on campus and in the community. Third, the RSU has consistently examined the actions of UMass in a global context, trying to ensure that the University acts in a socially responsible way. Fourth, no member has to subscribe to any particular ideology to be involved. These principles, however, have typically been coupled with other, seemingly contradictory factors. From the beginning, the RSU membership has been drawn primarily from white and middle to upper class students, while few of the people of color who have joined the group stayed active for long. The group has faced similar difficulties in developing women in leadership roles, even though the membership has been roughly balanced between men and women.

By 1980, the RSB had achieved a degree of stability on campus and a relatively high visibility. When the Revolutionary Student Brigade officially joined the Revolutionary Communist Party (changing its name to the Revolutionary Communist Youth Brigade), the UMass chapter was one of several that struck off on its own, adopting the name Radical Student Union. Despite this break with the national leadership, the RSU maintained some aspects of the earlier party-based organization, continuing to distribute revolutionary readings at meetings for future discussion, and having a secret collective into which only some members of the broader group were invited and which only a few even knew about. On its won, the RSU gradually evolved into a less doctrinaire organization, shedding the secret collective and affiliating with a variety of organizations. From 1981-1987, for example, the RSU joined the Progressive Student Network, a loosely linked coalition of campus-based student groups that sought to share resources and ideas and lend assistance when needed. They have subsequently worked with groups ranging from the American Friends Service Committee to the Students Coalition Against Nukes Nation Wide, United Students Against Sweatshops, Palestinian Action Coalition, the Five College Peace Network, and People for a Socially Responsible University.

On campus, the RSB and RSU worked on a long succession of issues. Among its mobilizations, the RSB organized protests around the 1978 death of Seta Rampersand, a black UMass student who was murdered in South Deerfield and whose death was never properly investigated. They were also active in protesting the decision at Kent State University to build a gymnasium on the site where four Kent State students were gunned down by National Guardsmen during an anti-war protest in 1970, organizing buses to take protestors to Kent State.

Among the most consistent threads in RSU activism has been a steadfast opposition to militarism and imperialism. Opposition to reinstatement of the military draft became a focus of concern after the return of Selective Service registration in the mid-1970s, and organizing around the issue of draft reinstatement has returned repeatedly, most recently in 2005, when a shortage of volunteers and wars on two fronts made it seem as if a new draft was imminent. Similarly, the RSU was deeply concerned with the realities of nuclear proliferation during the Reagan years. The Three Mile Island incident in 1979 highlighted the problems of nuclear power and the bellicosity of the Reagan administration led the RSB to join with national and local groups and organize around these issues through at least the early 1990s.

During the Reagan years, the RSU was active in opposition to American "imperial" expansion, and particularly to intervention in Central America, support for the Apartheid regime in South Africa, and the militarization of research on campus. In a series of demonstrations and marches, both locally and in Washington, D.C., RSU members protested the government's funding and training of death squads in Central America, the efforts of the Central Intelligence Agency to recruit on campus, and the use of Department of Defense funds in weapons research on campus. These protests came to a head with two major waves of sit-ins. In 1987, a group of students and community members occupied Memorial Hall, leading to sixty arrests, including Abbie Hoffman and former president Carters daughter, Amy. All were eventually acquitted. Two years later, students sat in at some of the labs in the Graduate Research Center where Defense Department research was taking place, and then moved to Memorial Hall, the chancellors office, and back to the Graduate Research Center. Organizing against apartheid, the RSU lobbied for UMass to divest from corporations based in and operating in South Africa. The April 1st Coalition took over the office of the Vice Chancellor of Student Affairs for four days and three nights ending the sit-in only when the university agreed to form a commission to study divestment. Off campus, RSU members have periodically sought to form coalitions with organized labor. In their first labor solidarity campaign in 1980, the RSU joined in support of a strike by nursing home workers at the Amherst Nursing Home.

After the high level of activity in the 1980s, the RSU has experienced a considerable drop off. Members joined in demonstrations against the Persian Gulf War in August 1990, including the protests at Westover Air Force Base, and they worked in solidarity to help win University recognition of the Graduate Employee Organization (GEO), and to convince UMass in 2000 to sign onto the Workers Rights Consortium, opposing the sale of sweatshop-produced apparel in the University Store. Efforts in the early 2000s to revive the organization met with mixed success, with some sustained protests against state budget cuts to education (the Save UMass campaign), against the wars in Iraq and Afghanistan, and on issues surrounding globalization. Members took part in protests against the Free Trade Area of the Americas in Québec in 2001 and Miami 2003, the latter notorious for violence. Following a campaign against labor and environmental abuses by the Coca Cola Corporation, the RSU almost disappeared, but was kept alive by several students until the fall of 2006.

The Revolutionary Student Brigade/Radical Student Union has managed to do something that few left-wing groups have: it has survived in more or less the same form it started with the same key principles and in the same physical space. Campus-based groups face difficulty in maintaining continuity as they lose members to graduation or the press of other concerns. The Radical Student Union remained active as of the 2006-2007 academic year, organizing several Critical Mass bike rides, protesting the honorary degree presented to Andrew Card, a senior official in the Bush White House, and attending the protests at the Bio International Convention.