Allen's victory in the European Tour event, part of the Players Tour Championship series, gave the 26-year-old from Antrim his first title of the season and the third of his pro career.
A packed house at the Lotto Arena saw him lift the trophy and earn 12,000 Euros and 2,000 ranking points.
World No 9 Allen started the final with a barrage of high scoring as breaks of 117, 102 and 106 put him 3-0 ahead. World No 1 Selby pulled one back before Allen made a 53 in frame five on his way to victory.
"It's been a long time coming because I've been struggling so far this season," said Allen, whose previous professional titles were the invitational Jiangsu Classic in 2009 and the Haikou World Open ranking event last March.
"I spent some time with my coach Terry Griffiths and he helped me get my mind right. I practised hard leading up to this tournament and I had a good attitude throughout. I'm pleased to be playing well again and looking forward to some big tournaments coming up."
Allen's triumph means that the 12 events on the World Snooker Tour so far this season have been won by 12 different players, and he added: "Snooker's more open now than it's ever been so there's no reason why I can't win more titles in the rest of the season."