SILVIS, July 15, 2013 (AFP) - The 19-year-old American outlasted Canada's David Hearn and Zach Johnson in the marathon playoff for his first career title.
"I just told myself 'this is it and go for broke'. I am fortunate it ended the way it did," Spieth said. "I owe 100 percent to my family.
"I don't even know what to say right now."
Spieth closed with his third consecutive six-under 65 to tie Hearn (69) and Johnson (68) at 19-under-par 265 at the Deere Run course.
Spieth, who was the American Junior Golf Association's player of the year in 2009, birdied five of his final six holes, including a chip in from a greenside bunker at the 18th.
Martin Flores fired an eight-under 63 during his final round to jump into a share of fourth place with third-round leader Daniel Summerhays (72) and Jerry Kelly (68) at 18-under.
Spieth, who played college golf at the University of Texas, earned a spot in the 2012 US Open as a replacement for Brandt Snedeker. He turned professional later that year.
He is the only golfer other than Tiger Woods to have won more than one US Junior Amateur, taking the title in 2011 and 2009.
Patrick Reed (67), Australia's Matt Jones (68) and J.J. Henry (69) shared seventh at 17-under 267 on Sunday.
Leading final-round scores on Sunday in the $4.6 million USPGA Tour John Deere Classic (USA unless noted, par-71, x-won in a playoff):
265 - x-Jordan Spieth 70-65-65-65, David Hearn (CAN) 66-66-64-69, Zach Johnson 64-66-67-68
266 - Jerry Kelly 68-64-66-68, Martin Flores 67-67-69-63, Daniel Summerhays 65-67-62-72
267 - J.J. Henry 68-65-65-69, Matt Jones (AUS) 66-65-68-68, Patrick Reed 67-63-70-67
268 - Steve Stricker 67-66-69-66, Jim Herman 66-68-67-67
269 - Kevin Sutherland 70-65-65-69, Steven Bowditch (AUS) 69-68-67-65, Nicholas Thompson 69-66-64-70