Willett began the day in the joint lead on 12 under with Matt Fitzpatrick but clinched victory by one shot ahead of his countryman on 17 under overall thanks to a bogey-free final round at the picturesque Crans-sur-Sierre course in the Swiss Alps.
The 27-year-old Sheffield golfer, who finished in a tie for sixth at the Open last Monday, picks up his first European Tour victory since triumphing at the Nedbank Golf Challenge in South Africa in December.
Tyrrell Hatton completed an all-English top three when he carded a best-of-the-day 62 to finish two shots off the pace on 15 under, with Sweden’s Pelle Edberg (14 under) and India’s Anirban Lahiri (12 under) fourth and fifth respectively.
Willett trailed Fitzpatrick by a shot on 14 under going into the back nine after he could only pick up shots at the fifth and ninth – but three successive birdies from the 13th saw him reclaim the lead and he never looked back.
Willett poured in a 25-foot putt up the green at the 13th to register his third birdie of the day and retake the lead on 15 under after Fitzpatrick, who carded four birdies and a bogey in an eventful front nine, added a second bogey on the par-three 11th to fall back to 14 under.
Fitzpatrick recovered well to pick up two consecutive birdies at the 14th and 15th holes to keep the pressure on the leader but Willett responded superbly when he converted a birdie putt from close range at the 15th to retain his one-shot lead.
The victory margin could have been wider had Willett sunk a 10-foot birdie putt at the 17th but it mattered little as Fitzpatrick failed to pick up shots at each of the final three holes.
Willett received a huge reception as he walked up to the 18th green and, needing a par to confirm victory, the Yorkshireman made no mistake from close range to earn his second Tour victory of the season.
Victory rounds off a very successful week for Willett following his exploits at St Andrews and he was more than pleased with his performance in Switzerland.
“I was very, very good. I kept very, very calm all day,” he told Sky Sports 4. “I just waited for the right things to happen and get on a run.
“Fitzy played some great golf, kept the pressure on all day. Right up until the last me and (caddy) Johnny (Smart) said, ‘Be prepared just in case – stranger things have happened in golf’.
“But I’m delighted. A great day today – I can’t be happier.”
Willett had been unwell when he posted a one-over-par 71 on Saturday but heralded his testing third round as being crucial to enabling him to earn victory 24 hours later.
“Yesterday was probably the key day – obviously we shot 63, 64 and 65 and then obviously we chucked in the 71,” added the world number 32, who could climb into the top 25 with his latest victory.
“But yesterday was a key day to stay in contention. It was a tough day – the wind was flapping and I wasn’t feeling the greatest – but I really knuckled down to grind out a one over par – it was probably the day that made it.”