Pillay was given out obstructing the field after a mistimed cover drive while facing Windies quick Jarion Hoyte.
Pillay edged the ball onto his pads, with the ball then dribbling towards the stumps, but stopping before hitting the woodwork.
Pillay, who had scored 47 runs, then tapped the resting ball with his bat, picked it up and threw it to Stewart (WATCH the incident in the tweet below).
Stewart then consulted the two standing umpires before the decision was referred upstairs.
According to ESPNcricinfo, third umpire Ranmore Martinesz made the decision in favour of the West Indies after a long wait and a number of replays.
Du Plessis was upset when he saw the incident and expressed his views on the matter on Twitter:
However, according to the letter of the law, the correct decision appears to have been made.This is a absolute joke...not in the spirit of the game .I have done this almost a 100 times. https://t.co/nX0KUJ9PI4
— Faf Du Plessis (@faf1307) January 17, 2018
Law 37.4, which deals with 'returning the ball to a fielder', states: "Either batsman is out obstructing the field if, at any time while the ball is in play and, without the consent of a fielder, he/she uses the bat or any part of his/her person to return the ball to any fielder."
The South Africans managed 282/8 off their allotted 50 overs, before bowling the Windies out for 206 to win by 76 runs.
South African batsman Jiveshan Pillay given OUT for handling the ball at the U19 world cup!
— UBET (@UBET) January 17, 2018
Thoughts? Out or not out? pic.twitter.com/U52mmSvl71