Victory gave Australia an unbeatable 10-4 points lead in the multi-format series, which concludes with a now ‘dead rubber’ Twenty20 in Cardiff on Monday.
England captain Charlotte Edwards elected to field after winning the toss and then saw her side restrict Australia to a seemingly meagre 107 for seven in 20 overs.
But England in reply collapsed to 87 all out — their lowest score in a Twenty20 international — with recalled seamer Rene Farrell taking three for 17 to win the player-of-the-match award.
Ellyse Perry and Farrell both took two wickets in an over as England slumped to 28 for five after their six-over power play.
Experienced England batsmen Edwards and Sarah Taylor both fell for eight and the hosts never recovered from seeing two of their best players fall so cheaply.
England came into this match with their confidence boosted by a seven-wicket win in the first Twenty20 at Chelmsford on Wednesday.
Australia, the reigning 50-over and Twenty20 world champions, had previously won the one-off Test and two of the three one-day internationals.
England knew they needed to win all three Twenty20 matches to level the overall score at 8-8 and so retain the Ashes by virtue of holding them prior to this series.
But having done well in the field at Hove, they panicked with the bat as Australia, well led by 23-year-old captain Meg Lanning, won their first Ashes in England since 2001.
“It’s nice to get our hands back on those Ashes,” said Lanning. “Everyone stepped up and players like Ellyse Perry and Rene Farrell led the way — they were superb.
“It’s an amazing feeling to win the Ashes in England, especially after such an all-round performance against a quality England side.”
A dejected Edwards said her side had been beaten by the better team.
“It’s disappointing, but Australia completely outplayed us in the series and they deserve to win the Ashes,” she said.
“I thought we did brilliantly with the ball and we should have chased 108, but we just weren’t good enough.
“We haven’t batted well throughout the series, really, and losing the Ashes will hurt for a while.
“But 2016 is a big year for us, including the T20 World Cup, and I think we have the players to bounce back.”