Battling with South African Richard Sterne for the coveted Omega Dubai Desert Classic trophy, the 38-year-old sank a 110-yard wedge from the rough at the 16th – amazingly his fifth eagle of the tournament.
With Sterne three-putting the hole, just as he had the previous green, the gap between the players went to four shots, and two final pars saw Gallacher win by three with a 71 – a tournament record-equalling 22-under-par.
"It's taken a long time, but it's even sweeter now," said the nephew of former Ryder Cup captain Bernard Gallacher after a win worth almost £263,000.
There were so many highlights from the Scot's performance it is hard to know where to start.
Gallacher matched his lowest Tour round with an opening 63 featuring a 175-yard seven-iron shot to eagle the sixth hole.
He got a ball stuck up a palm tree in his second-round 70, with a caddie climbing up a ladder to identify it, and then came a career-best 62 rounded off by sinking a 40-yard bunker shot for another eagle.
As for what happened on the 16th, he added: "I don't really know what to say. You need that little bit of magic."
Gallacher's first win came when he claimed the Dunhill Links title at St Andrews nine years ago, and this was his 404th career start.
He came into the tournament ranked 111th in the world, but should now qualify for the 64-man Accenture Match Play Championship in Arizona this month.
And, if he can make it into the top 50 by the end of next month, a debut in the US Masters at Augusta beckons.
Gallacher said: "It's the only major that I've not played. I sit with the kids and watch it every year, so it would be a dream."