The crash happened sometime after 3 p.m. near 4th Avenue South and South Spokane Street.
It's unclear exactly how the accident happened, but officials with the Seattle Fire Department say a 26-year-old bicyclist was hit by the semi and became stuck underneath the truck. The man was dragged for a short distance before the driver stopped.
Fire crews arrived on scene moments later and used industrial airbags to lift the truck and free the injured cyclist.
The man fractured several bones and suffered "road rash" in the wreck, but he was conscious and alert as medics drove him to Harborview Medical Center, according to fire officials. He is currently listed in stable condition.
The crash snarled traffic in the area as police investigated and cleared the scene.