Ko earned a score of 9.70 in the golf rankings released on Monday, barely ahead of 9.67 scored by Park In-bee of South Korea who had been the world's No. 1 since October last year.
Ko moved up one notch to the top spot as she tied for second place at the LPGA Tour's season-opening Coates Golf Championship in Florida, Sunday. Park tied for 13th.
Ko became the youngest to climb to the No. 1 spot at 17 years, nine months and seven days old.
The prodigious teen recently traded her glasses for contacts.
Stacy Lewis of the United States ranked third with a score of 8.83, followed by Feng Shanshan of China (6.40) and Suzann Pettersen of Norway (6.09).
Choi Na-yeon, also of South Korea, winner of the inaugural Coates championship, climbed five notches to 12th place, while compatriot Jang Ha-na who tied for second place with Ko at the championship moved up three notches to 18th.
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