He described his meeting with the Pope as "extraordinary," saying it happened before Francis left for Greece, where he's visiting refugees.
They met in a hallway at the Casa Santa Marta, the Vatican residence where Sanders and his wife spent the night, he said.
"It was an extraordinary moment. And I enjoyed the opportunity to chat with him," Sanders said.
He declined to provide details on what the Pope said, saying he'd rather keep it to himself.
" I just wanted to let him know how appreciative I was and the extraordinary role he's playing throughout the world in raising consciousness about massive levels of income and wealth inequality," he said.
Sanders said there were no photos of the meeting.
Speaking to reporters on the Papal Plane on the way back from Lesbos, Greece, Pope Francis said his handshake with Sanders was not a political act but simply good manners.
"It's called manners and not getting involved in politics. If anyone thinks that a greeting is getting involved in politics, then I recommend they look for a psychiatrist," the Pope said, laughing.
Asked about the meeting on Fox News' "Fox and Friends" Saturday morning, GOP front-runner Donald Trump suggested it wasn't a big deal.
"Five minutes sounds like 'Try and get me in to see him so I don't get myself embarrassed before I come back to New York,'" Trump said, referring to the Vermont senator's lagging poll numbers ahead of Tuesday's primary. "So a five-minute visit, you cannot do much in a five-minute -- after you say 'hello,' there's not time left."
'It was wonderful'
Columbia professor Jeffrey Sachs, who was at the meeting, said Sanders got word that the Pope wanted to meet at 6 a.m. before he left for Greece to meet with refugees.
"He wanted to greet the senator. It was wonderful," Sachs said. "They had a few minutes ... together."
Sachs said Sanders' message "about the morality of our economic life" resonated with people they met at the Vatican.
"That of course is a key theme and message of Pope Francis and I think there is a lot of resonance," he said.
Sanders takes message to Rome
Sanders landed in Rome on Friday morning, and traveled to the Vatican in a motorcade. His wife, Jane, four children and a few grandchildren traveled with him.
Sanders entered through the Perugino Gate of Vatican City, where he was greeted by a dozen or so expat supporters bearing signs reading, "Rome is berning."
Sanders began his 15-minute speech to the Pontifical Academy of Social Sciences with a show of admiration for Pope Francis, emphasizing their agreement on issues of economic justice.
"There are few places in modern thought that rival the depth and insight of the Church's moral teachings on the market economy," Sanders said, touching on a topic he has been speaking about for months during the presidential race.
His speech Friday largely echoed his usual remarks on the campaign trail in the United States: that wealth is concentrated in the hands of too few, and the middle class is disappearing.
"We can say that with unregulated globalization, a world market economy built on speculative finance burst through the legal, political and moral constraints that had once served to protect the common good," he said.
'Great fan' of Pope
After his address, Sanders walked through the gate and greeted onlookers before briefly speaking to the press.
Sanders had said before the trip he would be honored to meet Pope Francis.
But on Friday morning, Francis sent a note to conference attendees saying that would not attend because he is going to Lesbos early Saturday morning. The Greek island has become a focal point of the Syrian refugee crisis.
The self-described socialist Jewish senator and the leader of the Catholic Church make for odd bedfellows in many ways.
Sanders rails against "the establishment," and his platform includes support for abortion and LGBT rights, in contrast to positions taken by Pope Francis.
Still, Sanders has shown fondness for the Christian leader, saying recently he was "a very great fan" of the Pope and his messages, especially on the need to pay attention to the dispossessed and avoid the idolatry of money.
The Sanders campaign denied any political motivation behind the trip, saying the conference was an opportunity for the senator to spread his message of economic inequality.