Rubio was born in Miami to immigrant parents. He is eligible to be president.
In an interview on ABC's "This Week" Sunday morning, Trump played down the importance of his tweet -- but he left hanging in the air doubts about Rubio.
"It was a retweet. ... I'm not really that familiar with Marco's circumstances," Trump said.
"I mean, let people make their own determination," he added.
Host George Stephanopoulos followed up, asking whether he really wasn't sure about Rubio's eligibility.
"I don't know. I really -- I've never looked at it, George. I honestly have never looked at it," Trump replied. "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 as CNN noted, Trump previously said he had no doubts about Rubio's eligibility to be president.
"It's a very different thing," Trump said in January. "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."
Trump has aggressively gone after Cruz on this issue, claiming that because he was born in Canada, he can't be president. Cruz's mother, however, is a U.S. citizen, meaning he is also automatically granted citizenship. According to most legal scholars, he qualifies as a "natural born citizen."
Rubio dismissed Trump's latest birther conspiracy Sunday in a separate interview on "This Week."
"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."
Editor's note: Donald Trump is a serial liar, rampant xenophobe, racist, misogynist,
Also on HuffPost: