The guitar legend was performing his epic 'Moths to Flame' solo at the outdoor Snai San Siro Hippodrome in the Italian city when he slipped on his guitar pedal, which was soaking wet from the heavy rain.
Kirk laughed off the fall and posted a video of the moment on Twitter.
He captioned the clip: ''Ummm- I slipped on my wet wah pedal.
It rained so much I felt like I was playing guitar in the shower. #MetInMilan @Metallica (sic)''
The 56-year-old rocker can be seen really going for it with his playing, before landing on his his back with his legs flung in the air, before carrying on like a professional from the floor and getting back on his feet again.
The 'Enter Sandman' hitmakers are currently on their 'WorldWired Tour' and show no signs of slowing down and plan to keep going for at least 20 years.
The band - completed by James Hetfield and Robert Trujillo - have no desire to quit touring or making music like their peers Slayer and are feeling more ''energised and rejuvenated'' than ever before.
Despite wanting to keep the band active, drummer Lars Ulrich, 55, admitted he has ''no idea'' what Metallica will ''sound or look like'' further down the line.
The stickman said previously: ''A lot of people are retiring, and we feel very energized and rejuvenated. I mean, we wanna go [a] long [time].
''We hope we can get another 20, 25 years out of Metallica. I don't know what it's gonna look like, I don't know what it's gonna sound like, but that's in our heads.''
Insisting they are in it for the ''long game'', he added: ''We're not sitting there thinking retirement or this or that; we're sort of more the opposite. So I would say, for us, everything we do is kind of a long game.
''And back then, when you were 17, you only think of, like, the next five minutes. 'Where is the next beer? Wooh!' So that's kind of different mentality nowadays.''
Metallica feel like they are in a new chapter of their career because they have a whole new younger fan base.
He said: ''It's crazy. It's unbelievable. There's more younger kids at our shows than I can ever remember.
''Half the audience is under 20. And it also feels more like it's [a] 50-50 [split] between boys and girls now, which obviously [wasn't the case] 20, 30 years ago.''
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