The horrifying accident happened on Saturday at around 11.30am at Lightwater Valley theme park, in Ripon, England.
Riders on The Ultimate, which is Europe's longest coaster and can reach speeds of 50mph, were left stunned when the deer was reportedly decapitated, showering them with blood.
The ride cuts through around one-and-a-half miles of forest and, although protected by a fence, sometimes the wildlife manage to stray across.
A spokesperson for the theme park told a UK newspaper: "We can confirm a young deer did unfortunately get hit by The Ultimate train and was killed instantly.
"There is a large perimeter fence surrounding the theme park but occasionally wild animals do manage to stray onto the park during the night.
"After the incident The Ultimate was closed for 30 minutes whilst the ride was thoroughly checked and then normal operation resumed."
This isn't the first time a deer has been killed by the ride. Back in 1994, a deer was also hit by the coaster, resulting in a 12-year-old boy being taken to hospital.