Sanders, who broke away from the presidential campaign trail to travel to Rome, spoke of the five-minute meeting afterward in interviews with CNN and ABC, calling the pope "a beautiful man."
There were no cameras at the meeting, which was described as an encounter in a foyer at the pope's residence at Santa Marta, shortly before he departed for the Greek island of Lesbos to see the plight of refugees.
"I was leaving Santa Marta. He knew that I was leaving and he was kind enough to greet me," the pope later told reporters on the flight home from Lesbos.
"I greeted him and his wife and another couple who were with them and staying at Santa Marta. Nothing more," the pope said.
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"It was a matter of politeness and if anybody thinks that greeting somebody amounts to meddling in politics, they should go find a psychiatrist," he said.
Sanders has made attacks on Wall Street, the "billionaire class" and a growing rich-poor divide the core of his campaign, and he seized the opportunity today to identify with Francis's calls for greater economic equality.
"I just conveyed to him my admiration for the extraordinary work he is doing raising some of the most important issues facing our planet and the billions of people on the planet and injecting the need for morality in the global economy," Sanders told ABC.
He said he wanted Francis to know "how appreciative I was of the extraordinary role he is playing throughout the world in raising consciousness about massive levels of income and wealth inequality, greed such that the top one percent now owns more wealth globally than the bottom 99 per cent.