During a visit to the Vatican he emphasised "the common good" was a vital part of any economy.
Mr Sanders is standing on a left wing platform promising to redistribute America's wealth if elected.
"I'm so excited and proud to be here with other people who are trying to create a moral economy," he told reporters.
The Vatican had initially said he would not meet Pope Francis but the senator said he had a five minute meeting with the pontiff.
He told AP: "I told him that I was incredibly appreciative of the incredible role that he is playing in this planet in discussing issues about the need for an economy based on morality, not greed.
"It was a real honour for me, for my wife and I to spend some time with him. I think he is one of the extraordinary figures not only in the world today but in modern world history."
The Vatican stressed his invitation to the tiny city state was not a political endorsement.
Sanders supporters chanted outside the conference saying "Rome feels the bern".
He said inequality was causing the world "devastating" problems and that he admired the Pope's position on the issue.
Despite having a vital vote in the Democratic race in just a few days Sanders said he "could simply not refuse to attend".
He said: "Our youth are no longer satisfied with corrupt and broken politics and an economy of stark inequality and injustice."
