State Police said that 27-year-old Sean Moylan was walking his dog on the westbound shoulder of the Coos Bay-Roseburg Highway.
Moylan let go of the dog's leash at around 9:20 a.m. and stepped into the westbound lanes in front of a commercial truck, officials said.
The driver, Matthew Meyer, 42, of St. Louis, tried to avoid the man, however police said he didn't have enough time to slow down and stop.
Moylan was taken to Sacred heart Medical Center in Springfield with life-threatening injuries.
The dog was not hurt; Douglas County deputies drove the dog to a local animal shelter.
Police said the truck driver is cooperating with their crash investigation.
Moylan's last known address was in California.
On Monday morning, Moylan was listed in critical condition in the Intensive Care Unit at Sacred Heart in Springfield.