Broad sanctions against Iran have been lifted by the United States after years of negotiating, although a small number of more limited sanctions were imposed on Sunday evening.
The lifting of trade restrictions means Iran can re-enter the global oil market, its ships will be able to use foreign ports, and it will be able to access world financial markets.
In return Iranians, who are hoping the deal will boost economic growth, are giving up nuclear weapons capabilities.
“This is a good day,” said President Obama in a televised address from the White House. “We have a rare chance to pursue a new path, a different, better future that delivers progress for both our peoples and the wider world.”
The fresh sanctions introduced on Sunday, far narrower in scope than those lifted the same day, were because Iran violated a ban on missile testing in October.
Three Americans held captive in Tehran were also released as part of the increasingly friendly diplomatic relations between the two countries.
Jason Rezeian, a Washington Post journalist, and two others landed at Geneva airport on Sunday evening.
Armenian foreign minister Edward Nalbandian was positive about the opportunities which the lifting of sanctions might bring.
“Armenia is keen to deepen its mutually beneficial cooperation with friendly Iran,” he said.
Iran shares a land border with Armenia and Azerbaijan, as well as Afghanistan, Iraq, Pakistan, Turkey and Turkmenistan.
"Armenia will focus on the expansion of economic cooperation in the BSEC region, and will promote the agenda through existing mechanisms in the upcoming six months", Foreign Minister Nalbandian told the organisation's Ministerial Council at its meeting in Kiev.