Sim birdied the 18th hole to force a playoff, then birdied again on the first extra hole to deny McKibbin a victory at the Adriatic Cervia Golf Club.
The 21-year-old McKibbin started the day six shots behind the four players tied for first and teed off three hours before the final group began, but he finished with a perfect 65 to set his clubhouse goal at 10 under.
Even that didn’t seem enough when co-leaders Sim and Antoine Rozner finished at 13 under, and the pair eventually collapsed on the back nine.
Sim birdied the third, fourth and eighth holes but bogeyed the 11th, 14th, 15th and 17th holes to fall one shot behind, but he made birdie from 22 feet on the final hole and from about half that distance on the first extra hole.
Rozner advanced in the second round with just a birdie on the 18th hole, then shot a brilliant 62 on Saturday to get to 13 under after his third birdie of the day on the 12th hole, but then dropped five shots in the next five holes.
“It was really, really tough,” said Sim, who was playing in just his fourth race since undergoing hip surgery earlier this year.
“Everybody was hitting bogeys and double bogeys, the wind was swirling, the holes were tucked into the corners.
“Putting in the hole on the 18th [in regulation] That was one of the coolest moments in golf for me, and to do it again in a playoff was awesome.
“I love this sport. It's fun to work hard for it and if you get rewarded like this now, it's great.”
With no playoff in sight, McKibbin's caddie had left the course for a nearby train station, while McKibbin was relieved to know that he, along with American Sean Crocker, had earned two berths in the upcoming British Open at Royal Troon.
Crocker finished tied for third with Germany's Jannik de Bruyne but qualified on the basis of his world ranking.
McKibbin, who recently made his U.S. Open debut at Pinewood and played in the final round against world number one Scottie Scheffler, said: “I got a taste of what it was like to play at the U.S. Open a few weeks ago, so I'm looking forward to going to Troon. I'm excited.”
“I’ve never been there, but I went to a practice match there last time. [in 2016]… I don’t remember anything, but I was there.”
