The Americans beat Beijing bronze medalists Xue Chen and Zhang Xi 22-20, 22-20 to clinch at least a silver medal. The 2004 and '08 champions will play the winner of the late semifinal between Americans Jennifer Kessy and April Ross and the top-seeded Brazilians, Juliana and Larissa.
It was the 20th straight Olympic victory for May-Treanor and Walsh.
"We just defeated the best team in the world for the past year. These Chinese women are great athletes," Walsh Jennings said over the stadium sound system after the win. "We know we have this in us, we've just got to go execute."
Earlier Tuesday, Brazil's Emanuel and Alison defeated Latvia to advance to the men's gold medal game. They will meet the winner of the last match, between the Netherlands and Germany.
May-Treanor and Walsh Jennings had never lost an Olympic set before last week, when they beat Austria 2-1 in pool play. But they gave up the first three points of the semifinal, fell behind 13-8 and didn't hold their first lead 17-16 of the first set. China saved two set points before Xue put one into the net tape to give the first set to the Americans.
The Americans held a slim lead most of the second, but China took the lead 17-16 and forced the Americans to take a timeout. Trailing 19-18, May-Treanor ran far behind the end line to retrieve an errant pass and bumped it toward the net -- too close -- and forcing Walsh Jennings to go under the net, delicately bumping it over as she slid.
China saved one set point, but May-Treanor fisted one to the back line to set up a match point, then Walsh Jennings' block fell to the sand and she leapt into the air to celebrate.
In the early men's semifinal, the reigning world champions from Brazil beat Martins Plavins and Janis Smedins 21-15, 22-20 to clinch no worse than silver -- a third straight medal for Emanuel.