Airport spokesman Perry Cooper says the man, an employee of Prime Aviation Services, died after he was taken to Harborview Medical Center for treatment of multiple injuries.
The man, in his 40s, was driving a piece of equipment called a tug in one of the north cargo areas at about 6:20 p.m. when he crashed into a stationary forklift, Cooper said.
Cooper said the driver was thrown from the tug and landed next to the forklift.
He suffered numerous injuries, though Cooper didn't specify the nature of those injuries. The man was working alone and Cooper said several minutes passed before another employee noticed the crash.
Medic One was called to the airport and medics performed CPR on the driver before rushing him to the hospital.
Surveillance video of the incident shows the driver "slump in his seat" before the crash, which may indicate he suffered some sort of medical problem, Cooper said.
There was not an aircraft loading in the area at the time of the accident and operations at the airport weren't delayed.
The driver's current condition is unknown.