It was the Thai world champion's eighth consecutive loss against the world No.4, who was also world champion in 2011.
An error-strewn performance from the 18-year-old Ratchanok handed Wang a big lead at 18-7 and the Chinese proceeded to take the opening game without much trouble.
Ratchanok started off well in the second game by taking a 4-1 lead. But Wang rallied with some inspired defensive effortd to make it 5-5 and then raced into a 14-9 lead.
Ratchanok recovered to equal the scoreline at 18-18 and 19-19 before Wang took the next two points to wrap up the title in 46 minutes.
Ratchanok overcame defending champion Sung Ji-Hyun of Korea 24-22, 11-21, 21-17 in the semi-finals while Wang Yihan defeated compatriot Wang Shixian 21-19, 21-12.
In the men's singles final, world No.2 Chen Long of China proved too strong for world No.1 Lee Chong Wei of Malaysia as he beat the defending champion 21-14, 21-15 in just under an hour.
It was Chen's first win in the Korea Open while Lee was a three-time champion (2010, 2012, 2013).
In the women's doubles, fifth seeds Bao Yixin and Tao JInhua of China beat compatriots Luo Ying and Luo Yu 21-17, 21-15. Top seeds Mathias Boe and Carsten Mogensen of Denmark took home the men's doubles title after beating China's Fu Haifeng and Hong Wei 21-12, 21-17.
Top seeds Zhang Nan and Zhao Yunlei won the all-Chinese battle in the mixed doubles final against No.2 Xu Chen and Ma Jin 21-18, 21-18.