The victory also extended the US team's Olympic winning streak to 40 games, a run stretching back to 1992 in Barcelona when they won the bronze medal.
Australia more than held their own in the first half, riding the power of young 6-foot-8 (2.03 m) centre Liz Cambage who dominated the middle, scoring 19 points by halftime.
"We've played a lot of basketball in the last month with my team and I don't think anybody's played better against us than Australia did in that first half," said US coach Geno Auriemma. "That was an impressive display of basketball."
Beaten by the US team in the three previous Olympic finals, Australia looked bent for revenge and led by seven points during the second quarter and by four at halftime.
A defensive adjustment keyed a US second-half surge.
"We had a game plan and that was to stop Cambage and be more patient at the offensive end," said US center Tina Charles.
"We had to slow her down, stop her at the free throw line, don't let her run all the way to bucket, and don't let her get deep in the paint," Charles said of Auriemma's half-time advice.
The strategy worked and the US players ratcheted up the defensive pressure.
They finished the third quarter on a 10-3 run to take a 65-59 lead, then picked up where they left off and outscored Australia 8-3 at the start of the fourth quarter to build an 11-point lead.
Despite her imposing size, Cambage seemed to disappear and Australia wilted as the reigning champions built a 15-point lead at 81-66 and coasted home for another victory.
The 20-year-old Cambage, who during a preliminary round game became the first woman to dunk the ball in the Olympics, took only one shot in the second half and one free throw.
"The USA brought the defence, they shut us down," Cambage said. "They went to zone and that shook us up a little bit.
"I know I backed down a little bit, so I put a lot of that on me."
Auriemma gave his players the credit.
"I'm really really proud of my team and the way we responded in the second half and how we came back," he said.
Charles and Diana Taurasi shared scoring honours for the Americans with 14 points, while Sue Bird added 13.
Despite going scoreless in the second half, Cambage led the scoring with her 19 points and Lauren Jackson posted 14 for Australia, who fell to 0-16 against the United States in combined Olympic and world championship competition.
France made sure of winning their first Olympic women's basketball medal by beating Russia 81-64.
"We know that USA is so good," said back-up point guard Edwige Lawson-Wade, who was high scorer in the game with 18 points. "Every position, every player they have can play really well.
"To beat the USA you have to do the perfect game. Perfect in defence, perfect in offence. It's something that happens once every few years, but it happens. So anything is possible. So we're going to play that game to win."
France, who finished fifth in their only previous Olympic women's tournament at the 2000 Sydney Games, surged to a 24-15 lead in the first quarter after scoring the first eight points.
They built a 13-point lead in the second quarter before Russia closed an 8-2 run to make it 38-31 at half-time.
Russia continued their comeback after the break and, boosted by a three-pointer by Becky Hammon, closed within 42-40.
That seemed to fire up the French who rallied behind captain Celine Dumerc and Isabelle Yacoubou and finished the quarter strongly to carry a 59-51 lead into the final quarter which ended in a long celebration for the French.
They danced together in a circle on the court and sang 'La Marseillaise', the French national anthem, with their fans at the North Greenwich Arena, rejoicing with members of the men's team including Boris Diaw, Ronny Turiaf and Nicolas Batum.
Besides Lawson-Wade, five other French players scored in double figures in a balanced attack that has carried the unheralded team to a 7-0 tournament record.
"Nobody talks about us. We don't exist in the Olympic Village," said French coach Pierre Vincent. "The only way to exist is to win. I told the girls in the locker room, if we win, we will exist."
Hammon complimented France, but said a poor shooting performance in which Russia made little more than a third of their shots had ruined their chances.
"They're a very good team, they're playing very well right now. They're playing very confident," Hammon said. "We just didn't have enough offensive firepower.
"We've got good shooters but you got to make them when the lights come on."
Alena Danilochkina and Hammon shared scoring honours for EuroBasket champions Russia with 13 points.
The United States and France will play the gold medal match on Saturday when Russia and Australia battle for bronze
- Liz Cambage