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The 26-year-old birdied the final hole to back up last weekend's successful defence of the Alfred Dunhill Links Championship title with the third Tour win of his career.
For the second Sunday in succession Ross Fisher piled the pressure on his compatriot Hatton, but had to settle for second, tied with Thailand's Kiradech Aphibarnrat.
"It's amazing to win one but to win back-to-back and defend in two weeks is unbelievable," world number 22 Hatton told europeantour.com.
"Although I found it hard to believe, I tried to stay patient. In the end, I guess I got my reward.
"The end goal is to get as high as possible up the world rankings. Obviously the last two weeks have helped a lot."
Former Ryder Cup player Fisher set an Old Course record of 61 when finishing three strokes behind Hatton at St Andrews last weekend, and he fired a flawless eight-under 63 in Italy to set a clubhouse lead of 20 under par.
Hatton was tied with Fisher and Kiradech, for whom a double bogey on the 16th hole proved costly, standing on the 18th tee.
He hit his approach shot to 15 feet and drained the putt to avoid a play-off and continue his remarkable run of form, which now stands at four top-10 finishes on the bounce.
Overnight leader Matt Wallace finished two shots back in fourth after a 69, while popular Spaniard Miguel Angel Jimenez, 53, grabbed his first top-10 finish since 2015.
Home favourite Francesco Molinari faded to end four strokes adrift of Hatton, while world number five Jon Rahm carded an excellent 65 to finish on 15-under.
Leading final-round scores in the EPGA Italian Open at Golf Club Milano, Monza, on Sunday (Gbr & Irl unless stated, par 71):
263 - Tyrrell Hatton 69 64 65 65
264 - Kiradech Aphibarnrat (THA) 64 67 68 65, Ross Fisher 68 66 67 63
265 - Matt Wallace 64 65 67 69
266 - Marcus Fraser (AUS) 67 62 72 65
267 - Francesco Molinari (ITA) 64 68 66 69, Tommy Fleetwood 71 66 63 67, David Horsey 70 64 69 64, George Coetzee (RSA) 65 69 65 68
268 - Mike Lorenzo-Vera (FRA) 68 67 67 66, Marc Warren 66 69 67 66, Miguel Angel Jimenez (ESP) 70 67 66 65, Eddie Pepperell 64 69 69 66, Nino Bertasio (ITA) 68 67 69 64
269 - Maximilian Kieffer (GER) 68 66 66 69, Jon Rahm (ESP) 67 68 69 65, Alejandro Canizares (ESP) 66 70 65 68, Nicolas Colsaerts (BEL) 68 68 67 66, Benjamin Hebert (FRA) 70 68 67 64, Matthew Fitzpatrick 68 69 64 68
270 - An Byeong-Hun (KOR) 65 69 66 70, Wang Jeunghun (KOR) 68 67 68 67, Austin Connelly (CAN) 68 65 72 65, Jordan Smith 70 67 70 63, Gregory Havret (FRA) 66 69 70 65
271 - Alexander Levy (FRA) 68 69 66 68, David Drysdale 67 70 68 66, Paul Dunne 67 69 66 69, Jamie Donaldson 64 67 71 69
