Sciver-Brunt hit an unbeaten 111 off 99 balls, resuscitating a struggling England and leaving them needing 15 from the final over, a task that ultimately proved beyond them.
Australia's spinners tore through England's batting line-up, with Alana King and Ashleigh Gardner taking three wickets apiece, while Georgia Wareham also picked up a wicket.
"Both sides really hung in there and Nat Sciver-Brunt, yet again, played an incredible innings - I thought she was going to take it away from us at the end," Australia captain Alyssa Healy said.
"But full credit to our girls, they showed all their grit and determination again and we got ourselves over the line."
Earlier, Australia's total of 282 was powered by a clinical knock from Ellyse Perry (91), while Annabel Sutherland pitched in with 50 and Wareham scored a rapid 37 - smashing 26 off the final over of the innings from Lauren Bell.
The win at the Ageas Bowl gave Australia an 8-6 lead in the multi-format series, guaranteeing they will return home with the Ashes, but England can still level the series with victory in the final ODI on Tuesday.
"We still haven't played our best cricket which is a little bit scary at times but we're obviously really chuffed," Healy said.
"We came here to win the Ashes, we've retained them so far but there's still one game to go so we're ready to put our best foot forward again.
"But to know that the Ashes are coming home with us is hugely exciting. Full credit to the group for getting it done."
