Dustin Johnson celebrates winning the WGC Dell Match Play trophy
The world No1 overcame spirited resistance from Spain's Rahm to claim his third tournament victory in succession.
The 32-year-old never trailed at any point in his seven matches and was only taken beyond the 16th hole for the first time in a hard-fought semi-final by Japan's Hideto Tanihara.
But he was also made to work hard to secure the first prize of £1.3million in the final as Rahm fought back from five down after eight holes to take the contest to the 18th.
But there a loud noise, reported as a slamming portable toilet door, distracted him and a poor chip shot cost him a chance of forcing extra holes.
Rahm was just starting his backswing for the 80ft shot at Austin Country Club in Texas when a sudden noise reverberated across the green.
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Dustin Johnson celebrates after putting the winning shot in the WGC Dell Match Play
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The 22-year-old Spaniard continued with his swing but hit the ball fat and he reacted in disgust when it stopped barely halfway to the hole.
"That's absolutely why I missed it," said Rahm. "I lost focus with my head, chunked it and left it short. For that to happen is just unfortunate.
"I understand if I miss the shot, but if something in a moment that's so quiet happens it makes it feel a little worse because it's something that I can't control.
"It feels so bad because of all the work I did on the back nine to come from five down to almost have a chance.
"It's a lot of positives to take out of this week so I'll be sad for a couple of minutes but then I'll be fine. I'm really proud of what I did, it was a great week."
Dustin Johnson beat Jon Rahm by one hole to claim the title
Johnson and Tiger Woods are the only players with five or more WGC titles and although Woods won an incredible 18, the former world No1 failed to collect the HSBC Champions trophy in his only two appearances.
Johnson, who is favourite to win a second Major title in the upcoming Masters, added: "This was a tough one, a tough match against Jon.
"I gave him 10 when I three-putted but that was about the only hole I gave him, he played tough. He's a great player, he's going to be a great player out here for a long time.
"I've got a lot of confidence in the game right now, feel like I am driving the ball great, I'm wedging it well. I can work on the mid-irons a bit but we can work on those next week after I take a couple of days off."
America's Bill Haas beat Tanihara 2&1 in the play-off for third place, although Tanihara had the thrill of making a hole-in-one on the seventh.