Petrie had two seasons in Scotland's under-21 team, making his debut against the Irish in 1997 while he was playing in France with Colomiers. Petrie's Scotland A debut was in the 99–0 win over the Netherlands at Murrayfield in December 1999. He won his first cap in the second test on the tour of New Zealand in 2000, his first try for Scotland came in 31–8 November 2000 win against Samoa. He was given the captaincy by then Scotland coach Frank Hadden in 2005 leading Scotland to their first ever victory against the Barbarians.