It was not immediately known how serious the injuries were, but at least one victim was said to be in critical condition.
The 18 victims were all on the shuttle bus, according to Atlanta Fire spokesperson Janet Ward. Half of them were taken to Grady Memorial Hospital, and half were taken to Atlanta Medical Center, she said.
According to NBC Atlanta affiliate WXIA, the shuttle bus serves hotels north of the airport, and was headed to the airport when the crash happened.
Hartsfield-Jackson is the busiest airport in the world, averaging nearly 2,500 arrivals and departures per day.