Pinkfong and Baby Shark cheer on the Washington Nationals with fans of all ages ahead of Game 3 of the World Series. Picture: AP

It was practically the anthem for this year's World Series, with tens of thousands of Washington Nationals fans clapping in unison and belting out "Baby Shark, doo-doo doo-doo doo-doo."

In Lebanon, it became a rallying cry after a video of protesters singing to soothe a frightened toddler went viral. And in many other places, the earworm has drawn derision, with late-night comedian Jimmy Kimmel suggesting its creator should be jailed for life.

To the contrary, the catchy tune about a family of sharks has become so lucrative that the Korean family behind it is now sitting on a rapidly growing multimillion-dollar fortune.

Kim Min-seok co-founded closely held SmartStudy Co. in 2010, and five years later its children's educational brand, Pinkfong, released "Baby Shark." His father runs Samsung Publishing, which also owns part of the startup. 

Listen: Baby Shark trap remix

The family fortune, based on stakes held by Kim's immediate relatives in those two companies, is now about $125 million - much of it thanks to the song.

SmartStudy declined to comment on the family's wealth.

Shares of Samsung Publishing soared 89 percent the week the World Series began as local media reported on the song's surging popularity among USbaseball fans. 

The Kim family owns 63 percent of Samsung Publishing, which in turn owns 21 percent of SmartStudy. Kim also directly owns a 23 percent stake in the startup, which Bloomberg valued by comparing it to four publicly traded peers.

Kim, 38, hardly set out to write a hit global song. After working at gaming companies including Nexon and developing content for kids at Samsung Publishing, he co-founded SmartStudy to focus on the growing market for educational content for smartphones.

The app-to-video maker's early days were tough, but eventually grew faster as the Baby Shark video became a sensation, Chief Financial Officer Seungkyu Lee said in a January interview.