“This is such an unbelievable moment for me,” the 16-year-old McClatchie said.
“I’ve come close in the past, and to finally take it all the way to the win is just an amazing feeling.”
McClatchie overcame a late wobble at the 16th in regulation play with a birdie at next to close with a 68 and a share of the lead at eight-under-par 208 with Du Plessis, who birdied the 18th for a 69.
Having congratulated each other, the pair then had to put their off-course friendship aside and return to the 18th for a playoff.
Both players birdied the par-five on the first trip, but after his tee shot on the second try went left and ended up behind a tree in the thick stuff, McClatchie showed his mettle when he hit an audacious cut to 25 feet from the pin.
Meanwhile Du Plessis’s drive finished just off the fairway, and he found the greenside bunker on approach. The reigning Kenyan Amateur Open champion hit a great trap shot to 10 feet, but McClatchie dashed his hopes when he calmly drained the eagle putt for victory.
After a runner-up finish at the SA Boys under-19 and third at the SA Boys under-17, McClatchie believed it would just be a question of time before he could win a “major” junior event.
“You learn from all those events and you build up your experience and it prepares you for the moment when you have to stand up and be counted,” said McClatchie.
“I just told myself to calm down, to slow down my breathing and to draw on all my past experience.
“I think all those close calls definitely helped me when I stood over that final putt.”
Meanwhile Jovan Rebula closed with a 71 to take third place on five under 211, while Jason Froneman eagled the closing hole in regulation play to sign for a 70 and a share of fourth on 213 alongside Jaco Linde.
Linde was in running with Du Plessis and McClatchie through 12 holes, but a double bogey at the par-four 14th derailed his chances of joining the playoff.