The young Aussie beat Manchester United's Paul Scholes and Wes Brown, as well as team-mate Darren Ward, to scoop the award and earn a place in the inaugural FA Team of the FA Cup.
Cahill was one of four FA Cup Semi Final players shortlisted for the prestigious award, run in partnership with UMBRO, as Millwall went head to head with Cup Final opponents Manchester United.
He polled 47% per cent of the votes following the Semi Final ties with Lions defender Darren Ward a close second with 46%, whilst Wes Brown could only manage 5% and Scholes 2%.
Delighted by news of his victory, Cahill said: "I'm honoured that so many fans have voted for me as the FA Cup Player of the Round. To beat the likes of Paul Scholes and Wes Brown, not to mention Darren Ward, is a thrill. It was the highlight of my career to score the winning goal which took Millwall through to our first ever FA Cup Final. It's the sort of thing you dream about as a kid.
Although this award has been given to me, I want to accept it on behalf of the whole team, because it was a tremendous team effort which got us through. We are all really looking forward to May 22nd and being part of one of the greatest sporting occasions in the world."
Cahill receives an engraved silver salver as well as £3000 worth of UMBRO football kit and equipment for a school, club, or organisation of his choice.