The duo claimed their third consecutive Olympic Games title after they defeated number six seeds Jennifer Kessy and April Ross in straight sets.
The first set was evenly matched and was all-square at 13-13 but then the experienced May-Treanor and Walsh opened up a three-point gap as they looked to take a 1-0 lead.
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 finale
Misty 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.'
Earlier, Brazil's reigning world champions Larissa Franca and Juliana Silva won bronze after they came from behind to beat Chinese duo Xue Chen and Zhang Xi in a hard-fought match.
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.
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