Iden produced strong performances in the swim and cycle sections before clocking 2:36:15 on the run to chase down France's Sam Laidlow and clinch victory with an overall time of 7:40:24 on the Big Island.
Laidlow finished second and Iden's compatriot Kristian Blummenfelt, who won in 2021, claimed third place.
"That was so freaking hard," said Iden, twice winner of the Ironman 70.3 World Championships.
"The last 10 km, I was worried about the legend of the island killing me. Everything was going pretty smoothly up until I caught Sam.
"Then when I passed him the island really tried to put me down. I'm so proud of Sam and Kristian - even though Kristian should've come second."
American Chelsea Sodaro (33) stormed to victory in the women's event on Thursday, 18 months after giving birth to her daughter.
The Ironman comprises a 3.8km swim, 180km bike ride and 42.2km run.
The race was first held in 1978 to settle a friendly argument among Hawaiian endurance athletes as to who was the fittest.
