Tied with Harold Varner III and Belgium's Thomas Detry entering the round, Rahm took over on the back nine with birdies on four consecutive holes, giving himself enough cushion to withstand a bogey at the last.
His final round put him at 23-under for the event at Hong Kong Golf Club in Fanling, as he comfortably defeated runner-up Detry, who shot a 67.
Fellow Belgian Thomas Pieters (66) was a stroke behind Detry in third place, while Varner (69) finished in fourth at 18-under.
The potential Australian challenge never materialised as Lucas Herbert, only three shots off the lead overnight, ended up with a level-par 70 to finish on 14 under, tied for eighth place alongside his compatriot Elvis Smylie, who finished more strongly with a four-under 66.
Breaking a long title drought were the Dustin Johnson-led 4Aces, who grabbed their first team win in 974 days with their 58-under showing.
Rahm had started the season with back-to-back runner-up finishes before hoisting the Hong Kong trophy.
"Very relieving," said Rahm, who has won the past two season-long individual titles. "That's the only way I can describe it. I've been very ecstatic for wins in the past. This one just feels like a big weight off my shoulder. That's all I can say."
With the victory, Rahm snapped a streak of 26 events without an individual win.
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