Spinner Sunil Narine captured four wickets and Kieron Pollard smashed an unbeaten 47 as West Indies cruised to victory with 10 balls to spare in the rain-affected match at the Arnos Vale Ground in St Vincent.
The match was reduced to 40 overs a side after rain began falling shortly after the start.
The Australians, who won the first match by 64 runs on Friday, could only muster a modest total of 154 for nine, with David Hussey [ Images ] top scoring with 37 off 62 balls.
Shane Watson [ Images ] (25), Mike Hussey [ Images ] (24) and George Bailey (21) all made starts but no-one was able to go on make a significant score as Narine ripped through the middle-order, taking career-best figures of 4-27.
"We kept on fighting but we were a little short," Watson said in a post-match interview.
"Once the sun got on the wicket it seemed to settle a little more but that's the great challenge of the playing in the West Indies and we have to adapt a little better."
West Indies, set 158 from 40 overs to win under the Duckworth/Lewis method, made a terrible start to their reply when Kieron Powell was bowled on the first ball of the innings, shouldering arms to Brett Lee [ Images ].
The home side were still in danger when they slumped to 74-4 in the 19th over before Pollard took control, slogging four sixes in his unbeaten 47 and sharing a 64-run partnership with Dwayne Bravo [ Images ] (30) that turned the match.
"It's a good feeling," West Indies captain Darren Sammy said. "The team played much better today, the bowlers did well again and there was much more responsibility when chasing the runs.
"We were searching for a win and we are glad we got over the line."
S Watson c sub (Bishoo) b Sammy 25
D Warner b Roach 13
P Forrest c Sammy b Roach 0
M Hussey c Baugh b Narine 24
D Hussey b Narine 37
G Bailey c Samuels b Dwayne Bravo 21
D Christian run out 6
M Wade c Pollard b Narine 3
B Lee not out 11
C McKay st Baugh b Narine 6
X Doherty not out 0
Extras: (B-2, LB-3, W-2, NB-1) 8
Total: (in 40 overs) 154 for 9
Fall of wickets: 1-19 (Warner), 2-19 (Forrest), 3-46, 4-77, 5-109, 6-121, 7-136, 8-136, 9-154
Bowling: Roach 8-3-23-2, Dwayne Bravo 6-0-30-1, Sammy 4-0-19-1, Russell 6-0-14-0, Narine 8-1-27-4, Pollard 4-0-18-0, Samuels 4-0-18-0
West Indies: (target 158 Duckworth/Lewis method):
K Powell lbw b Lee 0
J Charles run out 26
M Samuels b Watson 20
Darren Bravo b Doherty 16
Dwayne Bravo run out 30
K Pollard not out 47
C Baugh not out 18
Extras: (B-4, W-2) 6
Total: (in 38.2 overs) 163 for 5
Fall of wickets: 1-0 (Powell), 2-42 (Samuels), 3-67 (Charles), 4-74 (Darren Bravo), 5-138 (Dwayne Bravo)
Bowling: Lee 8-1-37-1, McKay 8-2-16-0, Watson 7-1-28-1 (1w), D Hussey 3-0-13-0 (1w), Christian 5-0-26-0, Doherty 7.2-0-39-1.