This was the ninth match of the series played in the West Indies and Australia batted first scoring 175 runs for the loss of all wickets in 50 overs.
Skipper Dunith Wellalage made a notable contribution with five wickets for 28 runs in his 10 overs.
Chasing a target of 176, Sri Lanka reached the target in 37 overs for the loss of only six wickets.
Sri Lanka lost 4 wickets within the first 49 runs but skipper Dunith scoring 52 steered the team to victory.
He secured victory by building two successful partnerships with Anjala Bandara and Ranuda Somaratne.
Dunith who demonstrated his talent both in batting and fielding, was adjudged the man of the match.
The Sri Lanka Under-19 team has succeeded in winning both matches they played in the World Cup tournament.
Former Sri Lanka batsman Mahela Jayawardene is the advisor to the Under - 19 team.