And Rubio says it's just the latest in a line of Trump's "outrageous" comments designed to suck up media attention.
Rubio is a U.S. citizen who was born in Miami. He is, in fact, eligible for the presidency -- even though his parents migrated from Cuba.
But Trump stirred controversy on Saturday by retweeting a message suggesting that neither Rubio nor Texas Sen. Ted Cruz, who was born in Canada to a U.S. citizen mother, meet the constitutional requirements for the Oval Office.
In an interview on ABC's "This Week" with George Stephanopoulos, Trump said he didn't know about Rubio's case.
He dismissed the importance of his move on Twitter, saying "it was a retweet -- not so much with Marco," implying that his concern is with the eligibility of Texas Sen. Ted Cruz, who was born in Canada to a U.S. citizen mother and is a U.S. citizen.
"I mean, let people make their own determination," Trump said.
He said he doesn't know whether Rubio is eligible to run for president.
"I've never looked at it, George," Trump said. "I honestly have never looked at it. As somebody said, he's not. And I retweeted it. I have 14 million people between Twitter and Facebook and Instagram, and I retweet things and we start dialogue and it's very interesting."
But Trump saying he's unsure of Rubio's eligibility is an about-face from a mid-January interview with CNN's Jake Tapper
In that interview, Trump said that while he doesn't see Cruz as eligible for the presidency, he has no doubt of Rubio's eligibility.
"It's a very different thing," Trump said then. "He was born here. He was born on the land. Ted was not born on the land, and there's a very strict reading that you have to be born on the land."
Rubio hit back hard on Trump's insinuations on ABC's "This Week."
"Look, this is a pattern. This is a game he plays. He says something that's edgy and outrageous and then the media flocks and covers that and then no one else can get any coverage on anything else," Rubio said.
"And that worked when there were 15 people running for president. It's not going to work anymore. I'm going to spend zero time on his interpretation of the Constitution with regards to eligibility," he said. "And I'm going to spend all my time talking about what this campaign should be about."
Questioning an opponent's eligibility for the presidency is a common move for Trump.
In 2012, he repeatedly pressed for President Barack Obama to release his long-form birth certificate. Then, during his 2016 campaign, once Cruz narrowed the gap with Trump, Trump pointed to Cruz's Canadian birth and said he should secure a court ruling making his eligibility clear.