The 38-year-old Australian, also the 2008 and 2009 winner, finished in 8 hours, 3 minutes, 56 seconds.
“I wish I could tell you how it feels,” Alexander said. “It’s unbelievable I wish everyone in the world could feel what I felt in the that mile. It’s the finish I’ve been dreaming of.”
Alexander leapt across the finish line, collapsed flat on his back and received hugs from his family.
The previous course record was 8:04:08 by Belguim’s Luc Van Lierde in 1996.
Alexander lost to fellow Australian Chris McCormack last year.
“By any stretch, I didn’t think I had a bad race last year. I just got beat by three guys who were better on the day,” Alexander said. “Every year is different. It’s very dependent on the conditions. I’m very happy with the year I’ve had.”
The 50 professional men and 30 professional women were joined by more than 1,800 amateurs who tested themselves on the notoriously difficult Big Island course.
The triathlon started with a 2.4-mile swim in Kailua Bay. A 112-mile bike ride then took athletes north to the village of Hawi and back to Kailua-Kona.
One of the biggest challenges was fighting the wind, which howls across great fields of ancient lava flows and can seem to come from every direction simultaneously.
The race wrapped up with a full 26.2-mile marathon in and around the usually quiet seaside town.
The men’s and women’s champions each received $110,000, while the runners-up took home $55,000. The purse totaled $580,000.