The rain closed in with England on 254-4 chasing down 305 to win.
Matters looked gloomy for England, however, after they lost Phil Salt and Ben Duckett in the space of four balls in the third over, but Brook, who plundered 110 not out, and Will Jacks (84) took control.
England's victory means it is now 2-1 to Australia heading into the final two matches.
"It's nice to get the first one on the board and hopefully plenty more to come," said Brook after scoring his first ODI century.
"We just have to keep doing what we said we are going to do [at Lord's] and keep being positive, take the game to them and put them under pressure.
"That should stand us in good stead.
"With the bat myself and Will just tried to build a partnership and create that platform. We didn't have much of a chat between innings.
"I thought the pitch got better towards the end of Australia's innings. It was a simple message of go out there and play your own game."
Australia looked well set to wrap up a series-clinching win when they posted 304-7 before Mitchell Starc took those early wickets.
Captain Mitchell Marsh, though, said England merited their win.
"I thought we did extremely well to get the 300 with conditions early on, a lot of seam and it was hard work," he said.
"They got a really good partnership going, and in one-day cricket with a big partnership you can set the game up for yourself, [so] fair play to them."