"The deal makes our country and our world more secure," said Obama at the White House.
He rejected accusations that it was weak and challenged its detractors, including Israel, to offer a better alternative.
"There really are only two alternatives," he said. "Either the issue of Iran obtaining a nuclear bomb is resolved diplomatically, through negotiation, or it's resolved with force, through war. Those are the options."
His remarks came as the White House opened a diplomatic offensive to persuade members of Congress not to pass a motion rejecting the deal that was finalised in Vienna this week after months of negotiations.
Critics in Congress warn that the decision to lift the UN arms embargo after five years would trigger an arms race as Iran used billions of dollars of promised sanctions relief to fuel its policies of regional aggression.
Obama said sufficient arms control would remain, and Israel and the Gulf States would, with Western assistance, be able to shore up defences against Iran.
The US Congress now has 60 days to scrutinise the deal.
Other questions focus on the quality of the inspections regime and whether Iran will come clean to the International Atomic Energy Agency (IAEA), the nuclear watchdog, over the military aspects of its now-frozen nuclear programme.
Senior Republicans wrote to Director of National Intelligence James Clapper, saying they doubted Iran would deal candidly with the IAEA and asking for clarification of Secretary of State John Kerry's assertion last month that the US had "absolute knowledge" of the past military aspects of Iran's nuclear programme.
Obama dismissed concerns that Iran would have 24 days to hold off IAEA inspections requests, saying that even if Tehran did cheat, the combination of "snap-back" sanctions and the reduction of Iran's uranium stockpile would give ample time to act, militarily if needed.
The chairman of the Senate foreign relations committee, Republican Bob Corker, who negotiated with the White House to win the right for Congress to review the deal, said Republicans had grave misgivings about the motives behind the deal.
"The context [of the negotiations] is Russia obviously wanting to sell them arms, China wanting their oil, the European community wanting to do business with them and, candidly, an administration that wanted to build a legacy," he said.
Israel has also started its own fierce lobbying campaign to persuade members of Congress not to approve the deal when it comes up for a vote, probably in September.
The Israeli ambassador to the US, Ron Dermer, said the deal paved the way for Iran to build a bomb and that Obama was fundamentally mistaken in his belief that the agreement would make Israel and the Middle East a safer place - a view shared by Saudi Arabia and other Arab states.
5 things that aren't covered
Human rights
There is no obligation on Iran to soften its human rights record, release political prisoners, or reduce its regular use of the death penalty.
Western prisoners
Families of three Iranian-Americans held in Iran, including Washington Post reporter Jason Rezaian, hope their release may be hastened but there's no direct link.
Funding groups like Hamas
Iran may end up with more cash from released assets and oil sales to increase its support for outfits such as Hamas and Hizbollah described in the West as terrorists.
Syria
Iran may be more amenable to a deal to end the Syrian war, or other conflicts, but it may feel empowered to up its support for regimes the West does not like.
Personal freedoms
There's no suggestion Islamic laws, such as women having to cover up with the hijab, will be relaxed.
