Kevin Pearson, 52, from Cockermouth in the north of England, was sitting reading a book when the Smartwatch warned him he may be having a heart attack.
By coincidence, Mr Pearson was in hospital at the time taking his father to an appointment.
So he showed the nurses what his watch was telling him and he was quickly seen by doctors, who confirmed there was indeed something wrong and put him under observation.
The Apple Watch - which takes regular heartrate readings and feeds them to the phone - alerted Mr Pearson to the fact his heart was beating as fast as 161bpm (beats per minute) despite the fact he was relatively inactive.
It advised him to stay sitting while it monitored his heartbeat for the next few minutes, showing it rapidly drop as low as 79bpm and rise as high as 135bpm.
Mr Pearson told The Independent newspaper: "I felt nothing. My initial reaction was that there might have been a problem with the Watch. I felt absolutely fine ... I wasn't feeling any symptoms, such as sweating or anything like that.
"I said, 'It's possibly just my watch that's wrong but can you have a look?'"
Doctors told Mr Pearson he was indeed suffering from atrial fibrillation - a fast and irregular heartbeat - and after specialists took further readings and blood tests they confirmed he was having an "event".
While medics were unable to determine the exact cause, Mr Pearson's symptoms could have been an indication of a heart attack or blocked arteries.
He is now using his Smartwatch to monitor his heart so he can seek medical attention immediately if it happens again.
Mr Pearson said: "Even now, if I hadn't put my watch on, I wouldn't know if I was having a similar sort of event ...
"They're monitoring now, and I'll let them know if it ever spikes again. The watch is set up so that it will tell me if my heartrate ever goes over 120bpm.
"I've used my Apple Watch for calendar events, to complete its targets by exercising, and using it to lose weight. The heart rate wasn't really of any particular value, and I didn't even know it could alert you if it was too high."
Mr Pearson was so grateful that he has written to Apple CEO Tim Cook to thank him for helping to save his life.