Underworld received high acclaim from literary critics, particularly for DeLillo's prose and ambition. David Wiegand of the San Francisco Chronicle declaring it DeLillo's “best novel and perhaps that most elusive of creatures, a Great American Novel." Many have described the book as emotionally powerful.
Other critics, however, praised DeLillo's prose but found the novel overlong and argued it could have benefited from more editing. On Salon.com, Laura Miller wrote that “Nick's secret, the one that supposedly provides the book's suspense, proves anticlimactic."
In May 2006, The New York Times Book Review named Underworld as a runner up for the best work of American fiction of the previous 25 years.
The well-known literary critic Harold Bloom, although also expressing reservations about the book's length, has said Underworld is "the culmination of what [DeLillo] can do" and one of the few contemporary American works of fiction that "touched what I would call the sublime," along with works by Cormac McCarthy (Blood Meridian), Philip Roth (American Pastoral and Sabbath's Theater), and Thomas Pynchon (Mason & Dixon, Gravity's Rainbow and The Crying of Lot 49).