The 19-year-old left-armer wasn't included in Australia's original squad and was only brought to England as a "development player" for the first two Tests in a bid to gain experience.
But Agar, who has taken 31 wickets in 10 first-class matches for Western Australia, forced his way in ahead of off-spinner Nathan Lyon as the tourists specialist slow bowler.
Also in Australia's side was batsman Steven Smith, again not included in the original tour party, having been drafted in as cover because of concerns over captain Michael Clarke's back injury.
Ed Cowan, who this season has played county cricket at Trent Bridge for Nottinghamshire, was picked as Australia's No 3 having lost his opening spot after new coach Darren Lehmann went with Shane Watson and Chris Rogers as his first wicket pair.
There was no place in the side for batsman David Warner banned by Australia until this match after his Birmingham bar room attack on England's Joe Root last month.
England left out seamers Tim Bresnan and Graham Onions from their 13-man squad with fast bowler Steven Finn included in their XI.
Yorkshire batsman Root was opening in a Test for the first time having previously played as a middle-order batsman.
Ashes-holders England are bidding for their third straight series win over Australia -- something they last achieved in the 1950s.Teams:
England - Alastair Cook (captain), Joe Root, Jonathan Trott, Kevin Pietersen, Ian Bell, Jonny Bairstow, Matt Prior (wicketkeeper), Stuart Broad, Graeme Swann, Steven Finn, James Anderson
Australia - Shane Watson, Chris Rogers, Ed Cowan, Michael Clarke (captain), Phil Hughes, Steve Smith, Brad Haddin (wicketkeeper), Peter Siddle, Mitchell Starc, James Pattinson, Ashton Agar
Umpires: Aleem Dar (Pakistan), Kumar Dharmasena (Sri Lanka)