The Argentinian, 36, went into the final day three strokes adrift of Richard Bland and world number five Sergio Garcia, but ended up winning by one shot from that pair and Belgian Thomas Detry.
Romero, who has struggled with form and fitness in recent years, belied his lowly ranking with a fantastic performance to claim his first title on either the European or PGA Tours since 2008. Detry made the first move, with five birdies on the front nine to briefly take the lead.
A bogey on the par-four 10th put a halt to his charge, as the focus turned to the final group of Garcia and England’s Bland.
The overnight leaders started slowly, with Bland playing his first eight holes in one over, while Spaniard Garcia was even par.
But both sprung into action around the turn, as the Englishman poured in three consecutive birdies to the Masters champion’s two in three holes as the pair moved a stroke clear.
The previous four years had seen the winner score at least 17 under par at Eichenried, and Romero catapulted himself into a tie for the lead with six birdies in eight holes.
Detry also rediscovered his groove with birdies at 14 and 16, leaving four players tied atop the leaderboard on 16 under with the last group having four holes to play.
The tournament took a dramatic twist, as after Detry had failed to find a birdie at the par-five 18th and signed for a 66, Romero did just that to set the clubhouse target at minus 17.
Bland and Garcia fell back with late bogeys, meaning that they both needed an eagle on the 72nd hole to force a playoff with Romero.
Garcia narrowly failed to chip in with his third shot, before Bland’s 15-foot eagle putt slid past the right edge to confirm Romero’s unlikely victory.
Leading final round scores
271 - Andres Romero (ARG) 67-71-68-65
272 - Richard Bland 67-69-67-69, Sergio Garcia (ESP) 66-70-67-69, Thomas Detry (BEL) 65-71-70-66
274 - Rikard Karlberg (SWE) 67-69-72-66
275 - Tommy Fleetwood 67-71-70-67, Renato Paratore (ITA) 70-70-67-68
276 - Hennie Otto (RSA) 68-69-70-69, Nacho Elvira (ESP) 72-70-68-66
277 - David Lipsky (USA) 71-69-67-70, Henrik Stenson (SWE) 67-69-72-69
278 - Kiradech Aphibarnrat (THA) 67-74-68-69, Scott Henry 69-72-66-71
279 - Joost Luiten (NED) 66-73-69-71, Joakim Lagergren (SWE) 69-66-69-75, Jamie Donaldson 70-71-68-70, Dylan Frittelli (RSA) 72-71-66-70, George Coetzee (RSA) 69-70-70-70, Pablo Larrazabal (ESP) 71-71-68-69
