Knox holed from 40 feet for birdie across the 18th green in regulation to join Fox at the top of the leaderboard, the New Zealander then missing from eight feet for a birdie of his own to win the title.
Ryan Fox succumbed to some 18th-hole magic from Scotland's Russell Knox at the Irish Open, but still picked up a handy consolation prize.
Ryan Fox has fallen agonisingly short of his maiden European Tour title.
"But right now, I'm going to enjoy this victory and this tournament". Felt comfortable out there, and you know, it was close. I hit two great putts on 18 and one grazed the edge and one came back at me so I'm happy.
"All the practice, all the misses, all the bad moments are taken care of with putts like that", he said.
However, the four-time major victor slumped to an opening 80 and went on to miss the cut for the third year in succession, despite a 10-shot improvement in the second round.
"I remember thinking as soon as I hit it, "oh, I've hit it a bit firm", and it just started tracking and it dropped, unbelievable". I've done it before in the USA on the last hole, 12 feet.
"I'm looking forward to the challenge". Hopefully a few days to figure it out and see if I can play well at The Open. He played some magnificent golf but his putter was stone cold and he will head to Carnoustie desperately hoping to rediscover his touch on the greens. "The rough might be a bit more hard than here so we'll see what the conditions are like when we get to Carnoustie".
"That's why you play golf, to hole a putt like that on the last hole".
His closing 68 was one of the best rounds of the day, only five players recording lower scores including the eventual victor, Knox.
With an early eagle on the fourth hole, the Scot added birdies at both the sixth and 10th holes as Van Rooyen struggled with three bogeys in his opening nine holes.
That putt got Knox through the back door into the playoff where he holed a putt from basically the exact same spot for another birdie and his first victory non-WGC victory on the European Tour.
The duo finished a stroke ahead of Spain's Jorge Campillo, who fired a course-record equalling 65 as defending champion Jon Rahm hit an eventful 66 to share fourth with overnight leader South African Erik van Rooyen.