On what was a thriller of an afternoon, Webb, who had been three behind leaving the 10th green, was home in 33 to add a 69 to earlier scores of 70, 66 and 70.
The 36-year-old Australian could not have been more right with her Saturday-night prediction that it would be "a battle right to the end." Arimura, in spite of her relative inexperience of playing among the LPGA contingent, refused to go away and had a 25-footer at the last to force a play-off.
It may have been Webb's 51st title overall but she could not have looked more elated had it been her first which, for the record, was the 1995 British Women's Open championship at Woburn.
"It feels great," she said. "It was a bit of an up-and-down day so I feel very happy to be slinking away with a one shot win.
"Chie," she said, "handled herself really well and was in with a chance all the way. I know she's won in Japan but today would have been a different feeling. She really toughed it out."
