Rubio told reporters that he would not discuss the allegations contained in a Wall Street Journal story that said Guilfoyle had asked a donor and supporter of US President Donald Trump to pay a $US100,000 ($A144,000) credit card bill just before her confirmation hearing, promising him access to administration officials if he did.
He spoke on Thursday before leaving Greece, where he delivered a speech calling for European countries to return to what he described as their civilisational roots.
Rubio said he was aware of The Wall Street Journal's report, which was published late on Wednesday, but had no idea if it was true and he would not address it.
"You guys walk around thinking that because some source told you something and you reported it, it's the gospel. It's not the gospel. It's not the gospel for me. That's not the way we do things," he said at the foot of his plane as Guilfoyle, who was there to see him depart Greece, looked on.
At a joint news conference with the Greek foreign minister earlier in the day, Rubio said he would not address the allegations while visiting a foreign country, but added, "Suffice it to say our embassy here is doing a great job. Our ambassador has been a strong ambassador here."
The top two Democrats on the House foreign affairs and Senate foreign relations committees have called for an investigation into Guilfoyle, but Rubio would not say if he was prepared to open one.
Congressional Democrats already were looking into claims reported last month by the Wall Street Journal that Guilfoyle acted improperly while serving as the top US envoy to Greece.
The Journal reported that Guilfoyle told guests at a reception that the US could undermine the Greek government if it chose to, promoted the business interests of one particular Greek construction and energy company, and allowed its lobbyist into private diplomatic meetings.
Guilfoyle, a former Fox News Channel host and California prosecutor who was once engaged to Trump's son Donald Jr, has denied any wrongdoing and Rubio has defended her before.
