Chris Kris spotted the 30-foot-long, 50,000-pound fish as it swam near a boat off the coast of Florida, ABC News reported Monday.
The 19-year-old jumped off the boat and grabbed the shark's dorsal fin. He hung on for 9 seconds until the whale shark decided to dive.
A video of his ride was posted on YouTube.
Kreis said he decided to go for the Saturday ride because "I might not be able to do it ever again." He said he let go because "if something doesn't want you on it, you might as well just let go."
Whale sharks are not considered dangers to humans. While its mouth is about 5 feet wide, certainly big enough to swallow a person whole, it feeds on tiny plankton.
Riding the fish is not illegal, but critics contend human contact can potentially harm the sharks by removing their protective slime covering.
Still, Kreis said, "I would absolutely swim with another whale shark."