A fax machine, also called a facsimile , telefax, or telecopier, is an obsolete piece of technology which nevertheless appears remarkably often in the Left Behind universe. A fax machine uses optical scanning and low-bandwidth modems to create a reproduction of a document at a remote location, using standard phone lines. Computers built in the last decade of the 20th century often featured the ability to emulate a fax machine to send and receive documents; Cameron Williams' laptop that sees use in Left Behind and Tribulation Force has that capability.

In The Rising, a fax communicates to Jonathan Stonagal (via his secretary Fredericka) that Reiche Planchette was unable to find a suitable "Bearer" for Project People's Victory. Presumably, Stonagal was an early adopter of the technology.

In Tribulation Force, Cameron Williams notes, when he is transferred from New York City to Chicago, that by using a modem and fax he could file his stories from anywhere, in an attempt to minimize the effects of a "lateral promotion" that he felt was sidelining him. Throughout the events of Nicolae, Cameron is able to keep functioning as a journalist thanks to this technology.

In Soul Harvest, a fax machine is used by Cameron, Ernie, and Loretta to find Chloe Williams, since Global Community refugee shelters have standardized on it and they have a picture of Chloe that can be used for identification purposes.

Leonardo Fortunato faxes Chaim Rosenzweig during the events of Apollyon, indicating what the GCASA press release will say about the astronomical anomalies that have begun happening, and asking (or, rather, demanding) him to endorse it. It is not clear why a botanist, even a world-famous one, would be asked to comment on asteroid impacts.

Notably, in the Left Behind universe, the technology remains in use long past its real world expiration date, even through the first century of the Millennial Kingdom (Rayford Steele receives by fax news of Kenny WIlliams' possible defection to The Other Light, including a scan of the doctored letter that he supposedly sent to TOL leadership). Persistent use of obsolete technologies is noted to be a part of the theme of technological regression that Jerry B. Jenkins often pursues in his works.

Fax machines are primarily an office technology, and thus do not appear often in the kids' series. Judd Thompson Sr. is mentioned having a home office featuring a desktop computer and a fax machine in Facing the Future, and uses the phone attached to the fax to call Judd Thompson Jr. when the main line is unavailable; Mac McCullum notes in The Rise of False Messiahs that Chloe's International Commodity Co-Op routinely sends travel orders by fax, presumably because by that point the Global Community had upgraded to digital communications and would not monitor older technologies as thoroughly.