Helio Castroneves and Simon Pagenaud wrapped up the top five.
Power attempted to pass his team-mate with nine laps remaining but Montoya closed the door on the Australian, causing the two Team Penske cars to make contact.
“I saw him make the move but he was way too far [back] and I wasn’t going to give him the position,” Montoya told IndyCar’s website.
“If he was beside me I would have said, ‘OK, go ahead’. When I got to the turning point he wasn’t even close. It is a shame we touched, but it’s all good, it’s racing.”
Power added: “If I hadn’t damaged my [front] wing, maybe I would have had another shot.
“He was very strong on the front straight and my exits weren’t very good. That was really my only chance.
“I thought maybe I would catch him off-guard there. You don’t expect someone to pass there, so, yeah, I gave it a shot.”
“I was surprised at how aggressively he turned but he wanted to win the race and so did I.”
Montoya had not won in IndyCar since the Pocono 500 in July last year on his way to a fourth-placed finish overall in 2014.
The 39-year-old Colombian led for 27 laps on the street circuit around St Petersburg, while Power dominated the race, leading for 75 laps but failing to take the chequered flag.