The world number four, who enhanced his glowing status in the game by claiming the Open and US PGA Championship this season, follows in the footsteps of Nick Faldo who received the accolade in 1990.
The US PGA Player of the Year award, first presented in 1948 to American Ben Hogan, is based on a mathematical formula that weighs tournament wins, official money standing and scoring average.
Although there are four events remaining on the 2008 US PGA Tour, Harrington has accumulated a points tally of 116 which cannot be overhauled by his closest challengers, the PGA of America said.
World number one Tiger Woods, a nine times winner of the award, lies second in the points standings on 78 but had to shut down his season after winning the US Open in June to have reconstructive knee surgery.
Fiji's Vijay Singh is third on 62 points with American Phil Mickelson and Spaniard Sergio Garcia tied for fourth on 54.
Harrington successfully defended his Open title at Royal Birkdale in July before clinching the US PGA Championship at Oakland Hills the following month by two shots.
The Dubliner became the first European to win the US PGA crown in 78 years, emulating Scottish-born Tommy Armour who triumphed at Fresh Meadows in 1930.