Agar, 19, was presented with the baggy green by bowling great Glenn McGrath at Trent Bridge on the morning of the first contest between Australia and England.
The left-arm spinner has played just 10 first-class matches and only made the tour of India earlier in 2013 on an internship.
However, new coach Darren Lehmann made the first big call of his reign in charge of the national team by selecting Agar to face England.
Captain Michael Clarke said: "He’s bowled really well since he came on the tour."
Nathan Lyon is Australia's other spin option and has considerably more experience having played in 22 Test matches.