VATICAN CITY — Vermont Senator Bernie Sanders, a Democratic presidential candidate, met briefly with Pope Francis at the Vatican on Saturday morning before the pontiff’s trip to Greece.

“I conveyed to him my great admiration for the extraordinary work that he is doing all over the world in demanding that morality be part of our economy,’’ Sanders said during his flight back to New York from Rome. “We have got to move toward a moral economy, not simply an economy based on greed,’’ he added.

Sanders, who had attended a conference at the Vatican on Friday, said he had also thanked the pope for his encyclical last year calling for action on climate change.

Later Saturday, on his return from Greece, Francis described the meeting with Sanders as a brief encounter. Sanders and other participants in the conference were staying at the Casa Santa Marta, the Vatican City guesthouse where the pope resides.

“This morning when I was leaving, Senator Sanders was there,’’ Francis told reporters. “He knew I was leaving at that time and he had the courtesy to greet me. I greeted him, his wife, and another couple.’’

Francis said the two men shook hands, adding that he was simply being polite, not political.

“This is called good manners,’’ the pope said. “If someone thinks that greeting someone means getting involved in politics,’’ he added, laughing, “I recommend that he find a psychiatrist!’’

The meeting lasted about five minutes, said Jeffrey D. Sachs, an economic adviser to the Sanders campaign who was present.

Michael Briggs, a spokesman for the campaign, said no photos were taken, in accordance with rules at the guesthouse.

“The senator very explicitly said that this was not about politics,’’ he said.