Good defensive work by Walsh and a kill at the net edged the double champions closer to an early advantage, with a fault by Kessy handing them five set points.
Ross saved a point with an attack but she could only delay the inevitable, as May-Treanor fired her team to a first-set win.
Both teams fought hard to gain an edge in the second set, with Ross and Kessy pulling back their opponents with a well-timed ace to take it to 7-7.
However, three straight errors allowed May-Treanor and Walsh to pull away and they then powered to victory with four match points to spare.
May-Treanor, who was playing in her last match, was delighted to finish off with a third Olympic Games title.
This means a lot. It is my last match, but I'm OK with slipping away because this has been a special finaleMisty May-Treanor
She said: 'It means the world. We set our expectations really high as a team, as people and as players.
'This means a lot. It is my last match, but I'm OK with slipping away because this has been a special finale.'
Xue and Zhang eased to victory in the first set before the Brazilian pair stepped it up in front of a packed crowd to win their first Olympic Games medal.