TOKYO, April 20, 2014 (AFP) - Going out one stroke off the pace, the amateur player rolled in five birdies against one bogey for a round of 68 to win the Japan LPGA Vantelin Ladies Open with a total of 11-under-par 205 in the southern city of Kumamoto.
South Korean Lee Bo-Mee finished second, one shot back.
Katsu broke the record set by South Korean Kim Hyo-Joo when she won the Suntory Ladies Open in 2012 at the age of 16 years and 332 days.
"My body felt as if it did not belong to me. I think I made the round without thinking anything," said the pony-tailed player, 157 centimetres (5 feet 2 inches) tall.
"I got so nervous on the final hole but until then I wasn't at all," said Katsu, who enrolled in a high school in her home region of Kagoshima this month.
Katsu started playing golf at the urging of her grandfather when she was six.
She has competed in 11 Japan LPGA tournaments and her previous best finish was 12th.
Ryo Ishikawa holds the record for the youngest winner on the Japanese men's tour, taking the 2007 Munsingwear KSB Cup at the age of 15 years, 245 days.
"When I was tied for the lead after the first round, they told me, 'You are probably going to win'," Katsu said.
"I didn't think it would be possible but I tried to work hard anyway. I really didn't think I would win."