A spokeswoman for the Schlitterbahn Water Park in Kansas City said it was saddened by the death and the park had been closed pending an investigation.
The incident happened on Sunday on the Verrueckt ride, which drops users almost 169 feet (52 metres) at 65mph (105kph) in a three-person raft.
No details were immediately given about the circumstances of the incident.
However, Kansas police have said it is being treated as an accident.
"We honestly don't know what's happened," Schlitterbahn spokeswoman Winter Prosapio said. "That's why a full investigation is necessary. We have to understand what's happened."
The boy was identified by his family as Caleb Schwab, the son of a state politician, Scott Schwab.
The park's website says all users of the Verrueckt must be at least 54in (1.37 metres) tall and the combined weight of people in the raft must be between 400 and 550 pounds (181-249kg).
A 10-year-old boy has died on a waterslide, billed as the world's biggest, in the US state of Kansas.