Starting the day on 7-under par, Sim shot 2-under and showed his class on the 18th, rolling in a 12-foot birdie putt to finish 9-under and apply the pressure to New Zealand's Harry Bateman who, playing behind, came to the 72nd tee at 8-under.
Bateman would eventually close with a bogey and Sim claimed his first professional title since 2009 by one shot over another New Zealander, Kieran Muir.
"It's an incredible feeling, it's an emotional feeling, I never thought this might happen again and it's my first professional win in Australia, there's a lot to look forward to," Sim said.
Playing a group ahead of leader Bateman, Sim said he never looked at the scoreboard until an impressive wedge shot set up his look at birdie.
“I never thought I was a chance to win until really my wedge shot on the last and saw there was three or four guys on 8-under, I was just going about my own business.
"That might've been a little bit different if I was in the last group but I guess I had a little bit of freedom of not knowing so I just took it shot-by-shot."
Congratulations to the 2017 Isuzu Queensland Open Champion, Michael Sim #QldOpenOctober 29, 2017
The win now gives him the security of a spot full-time on the ISPS HANDA PGA Tour of Australasia until the end of 2018.
The 33-year-old will now take on the grueling six-stage Japan Tour Qualifying School under less pressure knowing he'll have a place to play in the foreseeable future.
"My priority now is to get the body ready, I've got the next six days off then I'm off to Japan for the third stage of qualifying," added Sim.
"Until this week, if I didn't get through in Japan I didn't know where I'd be, I'm not too high on the Australasian Order of Merit and I've got a really small Web.com category as a past champion, but there was still obviously doubt there.
"I think I've kind of relaxed in the last few weeks and just tried to play golf, if I was hitting a bad shot or miss a short putt I just move on.
“This game, you just don’t know what’s going to happen, I felt like I was on top of my game six years ago, top 50 in the world, I know I’m still a little far away from that but at least I know I can still do it.”
It's been a big 2017 for Sim who married his wife, Simone, earlier this year and they are expecting their first child in early 2018.
"It's going to be an exciting time for Simone and I next February, it's going to be a really happy time for our families and looking forward to taking the next step."
Fellow Western Australian Oliver Goss, Queenslander Damien Jordan and Bateman shared third place on 7-under.
Queenslander Douglas Klein was the best placed amateur finishing 5-under and T7.
For his win, Sim picks up AU$16,500 and moves to 28th on the ISPS HANDA PGA Tour of Australasia Order of Merit.
SIGNATURE HOLES - 7TH AT THE GLADES (QLD)
Want video tips delivered straight to your inbox? Subscribe to iseekgolf.com newsletters.