SEOUL (Reuters) - A chubby South Korean singer who has taken the pop world by storm, topping Apple's Itune downloads and getting 270 million Youtube views, has vowed to perform his "Gangnam Style" hit topless if it reaches number one.

The rapper who does the foot-stomping "horse dance" and goes by the name Psy, returned on Tuesday to the upmarket Seoul suburb that he put on the world music map.

"If it ranks number one in the Billboard chart, then I will perform 'Gangnam Style' topless in a place where everyone can watch," Psy told a news conference on his return from the United States.

Gangnam is the most upmarket neighborhood in the South Korean capital. Known as Seoul's "Beverly Hills", its streets are lined with designer-brand shops and trendy bars and restaurants.

Psy, who sums up his style as "dress classy, dance cheesy", has became a surprise hit in the United States where he was feted on chat shows and signed a deal with Island Records.

Psy, whose real name is Park Jai-sang, has always stuck out as an oddity compared with the svelte, well-scrubbed members of Korean girl and boy bands that have swept to fame in Asia and beyond.

After gaining recognition with a 2001 debut album, he landed in hot water several times. He says he was caught smoking marijuana and later had to repeat his mandatory military service after not taking it seriously the first time.

The 34-year-old was raised in Gangnam and went to the Berklee College of Music in the United States. He dropped out of a U.S. business school which his traditional parents hoped would prepare him for taking over the family business.

Referring to his kitschy image, he said: "Honestly, I like being 'second class'... I was born 'B' class."

His hit, released in mid-July, was intended to be fun, he said. Doing a bit of his horse-riding dance, Psy declared in English that his aim was to have "fun by music".

While he hasn't taken over the family business, his new-found fame seems to have helped it considerably.

The value of his father's technology company, D I Corp's, which is listed on the Seoul Stock Exchange, has doubled since July. Its market capitalization has surged to 113.5 billion won ($101.29 million) since he shot to fame.

Psy plans to release a new record in the United States in November.