The 50-year-old bicyclist was not wearing a helmet when he was hit near the intersection of Yale Avenue and Howell Street about 8:37 a.m., according to Seattle Fire Department spokesman Kyle Moore.
"He suffered a lot of injuries... that are pretty consistent with being run over," Moore said.
The man was rushed to Harborview Medical Center where he was listed in critical condition.
Police at the scene said it was unclear if the driver of the tandem dump truck even knew the bicyclist had been hit.
According to investigators, the front of the truck missed the bicyclist and he was hit back the back section of the truck.
Officers are looking for the truck, which officials said is blue and gray and was last seen taking the onramp to Interstate 5.