The 37-year-old seamer, the fifth most successful bowler in Test history with 602 wickets, told Sky Sports after stumps on the third day: "Tomorrow (Sunday) or Monday will be my last game of cricket.
"It has been a wonderful ride and a huge privilege to wear the Nottinghamshire and England badge as much as I have."
Broad, the son of former England opening batsman Chris Broad, added: "I am loving cricket as much as I ever have. I have always wanted to finish at the top and this series feels like one of the most enjoyable and entertaining I have been a part of."
Broad, appearing in the 167th Test of his career, made his debut in December 2007 against Sri Lanka in Colombo.
He has been half of one of Test cricket's most durable partnerships alongside fellow veteran fast bowler Jimmy Anderson.
I've been thinking about it for a while, a few weeks," said Broad, who has appeared in every home Ashes Test since 2009.
"England v Australia has always been the pinnacle for me."
Broad was two not out at stumps on the third day in England's second innings 389-9, with the hosts now 377 runs ahead as they push for a series-levelling win.
"I have loved the battles with Australia that have come my way and the team's way, I have a love affair with the Ashes and I think I wanted my last bat and bowl to be in Ashes cricket."
Broad, a four-time Ashes winner, added: "I told Stokesy (England captain Ben Stokes) last (Friday) night and told the changing room this morning, and to be honest it just felt the right time.
"I didn't want friends and teammates to see things that might come out. I prefer just to say it and give it a good crack in the Australia innings.
"It's been awesome. I don't think I will leave the game with any regrets. I will give it a red-hot go over the next couple of days."
Broad, who on Friday became the first Englishman to take 150 Test wickets against Australia, made his name in Ashes cricket with a return of 5-37 at The Oval in 2009.
And the most spectacular bowling performance of a Test career littered with dramatic wicket-taking bursts came when he took a remarkable 8-15 against Australia at his Trent Bridge home ground eight years ago.
'Hell of a career'
Australia off-spinner Todd Murphy paid tribute to Broad after Saturday's close.
"What you admire about a guy like that is that he charges in no matter what the conditions are and he fights all day and I think that's all you can ask from a cricketer and his record is unbelievable," he said.
"He's had a hell of a career"
Broad was also once an impressive lower-order batsman, making 169 against Pakistan at Lord's in 2010.
But he has never been quite the same with the bat since having his nose broken by a bouncer from India's Varun Aaron four years later.
At stumps on Saturday, Broad was batting alongside Anderson, the only non-spinner to have taken more Test wickets than his longstanding teammate, with 690.
Although several years older than Broad, the Lancashire swing bowler - 41 on Sunday - has insisted he has no plans to retire.
