Sato, who became the first Japanese driver to win the Indy 500 in 2017, crossed the finish line unchallenged on Sunday after team-mate Spencer Pigot crashed into the wall with five laps left, bringing out the yellow caution flag.
Scott Dixon, who led for 111 laps and was locked in a fight for the front with Sato, finished second.
The 43-year-old Sato, who made 90 Formula One starts between 2002 and 2008 and finished on the podium once, said he "never even imagined a situation like today".
"Look ... 40 years old, still driving, that's just living in a dream," Sato told reporters after the race.
"People say 2012 was probably my peak and the best shot ... we kept on going.
"We had a second chance in life. I say keep challenging, keep open the door, then you'll get eventually the chance.
"I never thought my racing life would go on any longer than Formula One. Now I'm competing here more than 10 years."
Sato, who became one of only 20 drivers to have won the Indy 500 multiple times, hinted that he would continue racing.
"You're always hungry to win, no matter who you are... There's just an energy that is Indy 500."
Pigot later tweeted that he was en route home, thanking the medical staff for caring for him.
With additional reporting from Reuters.