Matthew finished with a 9-under 279 for her fifth victory.
'I'm absolutely delighted,' Matthew said. 'I struggled a bit at the turn but the birdies on 12 and 13 helped to settle me down.'
Matthew had birdies on four of the first five holes to take a 5-shot lead.
'It's fantastic. I haven't won in a couple of years and there's a great field here this week so I'm just thankful to have won,' Matthew said.
Gustafson (68) and Diaz (68) tied for second place at 6-under, and Maria Hjorth (70) tied for fourth at 4-under with LPGA Championship winner Suzann Pettersen (71).
Defending champion Annika Sorenstam closed with a 68 for her best round of the tournament and moved from 21st to ninth.
After playing 43 straight holes without a birdie, the Swede made six birdies in the final round.
'It was different today,' Sorenstam said. 'I hit the fairways, and if you do that you also hit the greens, and you get birdie chances. I'm happy with my score today, but I have to work harder.'
A Hall of Famer who has won 10 majors and 84 tournaments worldwide, Sorenstam is in the midst of the most disappointing season of her career. She had a second place-finish in March, then she didn't make the top 10 again until July. She sustained a back injury in April and was sidelined for two months.