Constructed in 1985 and renamed in 1997 in honor of former Colby president William R. Cotter and his wife, Linda, Cotter Union is the social focus of Colby’s residential life. Cotter Union includes the Page Commons Room (for parties, lectures, concerts, and other large-scale events),  a coffee shop, the student post office, conference rooms, lounges, and the Office of Campus Life. The Pugh Center houses offices for the Student Government Association and multicultural organizations on campus, as well as meeting space for social and cultural programs. The Bridge Lounge connects one main wing with the Marchese Blue Light Pub, a game room, and an outdoor terrace. Opening in August of 2007, The Pulver Pavilion houses the Joseph Family Spa (which serves, drinks, short-order meals, and snacks), as well as the living room of the campus. 

Rooms in Cotter Union may be reserved using the Colby Web event placement form or by contacting the Office of Campus Life. Use of tables or other space in the lobby should be arranged through the Office of Campus Life. Posters and other notices may be placed on bulletin boards located throughout the union, and will be removed after the date of the event. Notices and posters will be removed from doors, glass, and walls and will be re-posted on bulletin boards.