Yamashita assumed control of the tournament with a seven-under-par round of 65 on Friday morning and led from that moment on.
She briefly shared top spot with playing partner A Lim Kim early in her final round, but the South Korean’s birdie at the second was quickly followed by consecutive bogeys.
Hull, who propelled herself into contention with a six-under 66 on Saturday, quickly emerged as the main challenger to world number 15 Yamashita.
They both played their first nine holes in three under par, meaning Yamashita reached the turn three shots clear.
Hull refused to relent and continued to attack, closing the gap to one shot until a costly trip to a fairway bunker on the 16th.
A couple of groups further back, Yamashita stayed remarkably composed and was able to limit the damage of her rare errors – superbly saving par with a lengthy putt at the 14th.
With the knowledge that a bogey on the par-five 18th would be enough for victory, there were some nervous moments when Yamashita found the rough with her first two shots – before a safe chip onto the green set up a par that confirmed she would be the third Japanese winner of the Women’s Open.
Speaking via a translator at the trophy presentation, she said: “To win such a historic tournament in front of all these amazing fans is such an incredible feeling.
“To be part of such a moment in history is something very special,” added Yamashita – who collected a winner’s prize of $1.462m (£1.1m).
