Kazuki Takahashi, whose popular manga comic series Yu-Gi-Oh! became a global franchise including TV shows, films and a hugely successful card game, has been found dead in the sea in Japan.
The 60-year-old died after an apparent snorkelling trip near the southern Okinawa Island, the coastguard said.
He was found off the coast wearing an underwater mask, snorkel and fins, an official for the city of Nago added.
"We are investigating the case as both a possible accident or crime," he said.
Yu-Gi-Oh!, which means King of Games, is about spiky-haired teenager Yugi, who receives an ancient puzzle that awakens in him the alter ego of an Egyptian pharaoh. He then solves conflicts using games.
It was published in Japanese comic magazine Shonen Jump between 1996 and 2004, and in 1999 was turned into a real-life trading card game.
A decade later, the game gained the Guinness World Record for the best-selling trading card game in history with sales of more than 22 billion cards, according to manufacturer Konami. The franchise also grew to include video games, books, toys and figurines.
Takahashi was found about 300m offshore, public broadcaster NHK said. He was found on Wednesday and identified on Thursday.