Rain delayed the scheduled 09.30am local (1330 GMT) start time but with the match due to get underway at 12.15am (1615 GMT) is reduced to 38 overs a side.
Australia were bidding for a third straight title and fourth in all, both unprecedented achievements in the nine-edition history of the World Cup.
They came into this match on the back of a record run of 28 games unbeaten in World Cup cricket, including 22 straight wins.
Australia were unchanged from the side that beat South Africa by seven wickets in Wednesday's semi-final in St Lucia.
This match was fast bowler Glenn McGrath's last before retirement. The 37-year-old was appearing in his fourth successive World Cup final, having also been on the losing side when Sri Lanka beat Australia in 1996.
Ahead of the Final, he had 70 World Cup wickets; his 25 to date in the 2007 edition was already a record for a single event.
Sri Lanka, whose team contained three survivors from the side that won the World Cup 11 years ago (Sanath Jayasuriya, Chaminda Vaas and Muttiah Muralitharan), were also unchanged following their 81-run win in Tuesday's semi-final against New Zealand in Jamaica.
Fast bowler Dilhara Fernando, whose five overs against the Black Caps cost an expensive 45 runs, retained his place despite reports from Sri Lanka he would be dropped.
Australia beat Sri Lanka by seven wickets in Grenada when the sides last met in a Super Eights, second round, World Cup match on April 16.
West Indian umpire Steve Bucknor extended his own record by standing in a fifth successive World Cup Final.
Australia: Matthew Hayden, Adam Gilchrist (wk), Ricky Ponting (capt), Michael Clarke, Andrew Symonds, Michael Hussey, Shane Watson, Brad Hogg, Nathan Bracken, Shaun Tait and Glenn McGrath. Coach: John Buchanan.
Sri Lanka: Upul Tharanga, Sanath Jayasuriya, Kumar Sangakkara (wk), Mahela Jaywardene (capt), Chamara Silva, Tillakaratne Dilshan, Russel Arnold, Chaminda Vaas, Dilhara Fernando, Lasith Malinga and Muttiah Muralitharan. Coach: Tom Moody.
Umpires: Steve Bucknor (WI), Aleem Dar (Pak) and Third Umpire: Rudi Koertzen (RSA).
Match Referee: Jeff Crowe (NZ).