The 2012 IndyCar season champion added: “This is as big as the championship for me. I’ve been watching this race since I was sitting on the floor in diapers.”

Hunter-Reay, who started 19th in the field of 33, led off a restart with six laps remaining after the race was halted to clear debris off the track.

Castroneves passed for the lead on the inside entering turn one with five laps to go, but Hunter-Reay took the lead back in the third turn of lap 197 with a daring inside move.

Castroneves, trying to become only the fourth man to win a record four Indy 500s, went to the outside to reclaim the lead on the first turn of lap 199.

But Hunter-Reay went outside on the front straightaway as the white flag waved to signal the start of the last lap, passed Castroneves just before the first turn and held off a final pass attempt at the finish line to win the biggest of his 13 career IndyCar titles.

“It was a fantastic fight to the finish,” Hunter-Reay said. “We all raced each other clean but really hard. I was just biding my time and waiting for the right opportunities.”

Castroneves could only smile and shake his head.

“It’s a shame. So close,” he said. “I did everything I could do.

“It was a great fight. I was having a great time. Unfortunately, second, it’s good, but second sucks.”

US racer Marco Andretti, the grandson of 1969 Indy 500 winner Mario Andretti, could not end the family’s 45-year Indy driving jinx, settling for third, .3171 of a second behind his triumphant teammate.