Yamashita carded a two-under par final round to hold off a strong challenge from England’s Charley Hull on the Welsh links course.
She finished on 11-under par for the tournament, with Hull and Japan’s Minami Katsu ending in a tie for second place after final rounds of 69 left them on nine under.
Just a day after turning 24, Yamashita became the third Japanese woman to win the British Open after Hinako Shibuno in 2019 and Ayako Okamoto in 1984.
She joyously swigged from a bottle of champagne on the 18th green before wiping away tears of joy after sealing her victory.
“To win such a historic tournament in front of all these amazing fans is such an incredible feeling,” Yamashita said.
“The course is set up to be very difficult but also in a brilliant condition,” she added. “The amount of people supporting me today really pushed me towards the victory and this is for them.”
Yamashita’s previous best performance in a major was a tie for second place at last year’s Women’s PGA Championship.
She survived a stern test of her temperament on the last day after resuming with a one-stroke lead over South Korea’s Kim A-lim.
Having posted a bogey-free 65 on Friday, she had carded a two-over par 74 marred by erratic driving off the tee and struggles on the greens in the third round.
Yamashita recovered her composure on Sunday as she embarked on a relentless march to the trophy.
Prior to this year, Yamashita’s highest finish at the Women’s British Open was 13th in 2022, but she mastered the Porthcawl course in emphatic fashion.
“To be part of such a moment in history is something special,” she said. “Being my first win is something very special and to celebrate with everyone is an amazing feeling.” (AFP)