Park began the day three shots off the lead but carded an eagle, seven birdies and two bogeys for a closing 65 to finish 12 under par, three shots ahead of compatriot Jin-Young Ko.
Joint overnight leader Ko, who was looking to win on her first major start, enjoyed a four-shot lead after following an eagle on the seventh with birdies on the eighth and 10th.
However, as Park charged through the field with four birdies in a row from the seventh and an eagle on the 14th, Ko three-putted the 13th and ran up a double bogey on the 16th.
“It’s the greatest day of my life yet for sure,” said 27-year-old Park, who has now won six of the 14 majors she has contested since 2013. “I played great golf out there.
“Even though I made a couple of early mistakes, I was able to hold on really well on the back nine. The golfing God was on my side for sure.”
The British Open was the only one of the ‘traditional’ major championships that Park had yet to win, although the Evian Championship only became the fifth major in 2013, a year after Park won what was then called the Evian Masters.
Thailand’s Kiradech Aphibarnrat held his nerve to win his second European Tour title of the year, beating Robert Karlsson in a hard-fought final of the inaugural Saltire Energy Paul Lawrie Matchplay.
Aphibarnrat, who ended hopes of a home victory by knocking out Marc Warren in the semi-finals, was three up with four holes to play but lost the 15th, 16th and 17th and saw Karlsson fire his second shot to the 18th to 10 feet.
However, the 26-year-old produced an even better approach from heavy rough and, after Karlsson dragged his birdie attempt wide, holed from four feet for victory and the first prize of £122,000.
Finland’s Roope Kakko fired a sparkling final-round 63 to secure the European Tour Madeira Islands Open by a convincing three-shot margin.