Led off by Mollie O'Callaghan and brought home by Titmus, with Lani Pallister and Brianna Throssell in the middle legs, they touched in an Olympic-record 7min 38.08sec.
The Katie Ledecky-led United States (7:40.86) took silver and defending champions China (7:42.34) bronze.
It avenged Australia's failure to win gold in Tokyo when China set a new world record to stun both them and the Americans.
"It was really fun. I feel like a bit of redemption for us," said Titmus, the 400m champion, who came second in the 200m final behind O'Callaghan.
"Tokyo was definitely not the result we wanted and I personally wasn't happy with how I performed in the relay in Tokyo.
"So I felt like I put pressure on myself to lift this team. I feel like I have a role to play in this team and do the best job I possibly can. And I think I did that tonight.
"I'm proud that they had faith in me and in the back to put me last and get the job done. But very, very special up there on the podium with the girls."
They got a flying start from O'Callaghan, who clocked 1:53.52 to give them the lead, but it was Titmus who sealed the deal, coming home in a sizzling 1:52.95 to leave the USA and China in her wake.
Their time at La Defense Arena was second only to their own world best, set at the 2023 world championships.
"It was amazing to be able to swim alongside these girls, I say over and over again it's a privilege and it's always exciting to compete for Australia," said O'Callaghan.
"There's not many words I can say because my brain is kind of going everywhere and I'm just really excited."
