The fraction of a second was the difference between a spot on the podium and missing out all together with Teagle officially handed a third-place finish for the event.
"Seeing how he was wrong, unconsciously I stopped. He deserved it," 21-year-old Spaniard Mentriga said after the race.
He told Eurosport: "When I saw that he had missed the route, I just stopped. James deserved this medal. He didn't notice the signs or they were misaligned. I don't know, but the second time I would have acted the same".
The classy display of sportsmanship has been praised across the sporting world.
The runner who won the Santander Triathlon, Javier Gomez Noya, described Mentriga's sporting display as "the best in history".
Other international sporting names described Mentriga's act as the definition of sportsmanship.