Trump is also urging Congress to reform U.S. law so the president is no longer required to declare every 90 days whether he thinks Iran is in compliance, or to renew sanctions waivers every 120 or 180 days.
He paired Friday's concession with other, targeted sanctions on Iran for human rights abuses and ballistic missile development.
The 2015 deal, known as the Joint Comprehensive Plan of Action, was signed by the United States under the previous administration of president Barack Obama as well as Iran, China, Russia, Britain, France and Germany. But he said he would work with European allies to remove so-called "sunset clauses" that allow Iran to gradually resume advanced nuclear activities in the next decade.
On Friday, the White House said that Trump would waive sanctions against Iran to keep the deal alive, but it would be the "last time" unless the United States and Europe reach an agreement in the next 120 days to strengthen the landmark agreement.
Trump on Friday extended the waivers of key economic sanctions that were lifted under the agreement limiting Iran's nuclear program.
The president's proposals to "fix the deal's disastrous flaws" include Iran's agreement to open all sites immediately to global inspectors and an assurance from Tehran that it will never develop a nuclear weapon.
Iran has said it will not accept any changes to the terms of the 2015 nuclear deal, which eased sanctions on the country in exchange for Tehran halting uranium enrichment.
"This is a last chance", the USA president said in a statement on Friday.
But Trump, who must meet regular deadlines to endorse the deal every 90 days and approve sanctions waivers every 120 and 180 days, has railed against the agreement. Widespread protests in Iran over the last few weeks have focused on the country's faltering economy, however, as well as high unemployment and government corruption.
Trump is required to renew the existing deal every 120 days under USA law.
European allies had strongly urged the White House to stick with the nuclear deal, saying a decision to abandon it would strain the transatlantic partnership.
Trump's expected announcement comes a day after Britain, France and Germany called on Washington to uphold the nuclear deal.
French President Emmanuel Macron called Trump on Thursday and stressed France's determination to see "the strict application of the deal and the importance of all the signatories to respect it".