Unbeaten in two previous starts over fences, Nicky Henderson's dashing grey was the market leader and travelled powerfully throughout in the hands of Barry Geraghty.
Despite making a jumping error heading down the back straight, the seven-year-old was still cruising heading down the hill and was left in front after Overturn blundered his way over the third-last.
There was still time for a scare for odds-on backers, though, as outsider Baily Green tracked him into the straight.
But Simonsig found enough after the final fence to seal a two-and-a-quarter-length triumph.
Geraghty said: "He rallied really well, in fairness to him."
Simonsig was already a previous Festival winner, having claimed the Neptune Investment Management Novices' Hurdle 12 months ago, but had not run over fences in public since December.
Overturn, a winner of numerous big races on the Flat and over hurdles, had won each of his first three starts over fences.
And while he went with his usual enthusiasm for a long way, Simonsig appeared to have his measure when a bad mistake stopped him in his tracks.
The most disappointing horse in the race was Willie Mullins' Arvika Ligeonniere, who travelled and jumped soundly for the first half of the contest before stopping very quickly at the top of the hill.
Geraghty said: "It was harder work than I thought. The ground is dead, and maybe that didn't help.
"He missed one down the back and that was unlike him.
"He was very good at the second-last and tight at the last.
"It didn't set the headlines as we thought he might, but he was very good."