"I'll be involved in those talks, indirectly. And they'll be very important," Trump told reporters aboard Air Force One.
Tensions are soaring ahead of the talks, with the US deploying a second aircraft carrier to the Middle East. The US military is preparing for the possibility of a sustained military campaign if the talks do not succeed, US officials have told Reuters.
Asked about the prospects for a deal, Trump said Iran has long sought a tough posture in negotiations but learned the consequences of that approach last northern summer when the US bombed Iranian nuclear sites.
Trump suggested Tehran was motivated this time to negotiate.
"I don't think they want the consequences of not making a deal," Trump said.
Prior to the US strikes in June, US-Iran nuclear talks had stalled over Washington's demand that Tehran forgo enrichment on its soil, which the US views as a pathway to an Iranian nuclear weapon.
"We could have had a deal instead of sending the B-2s in to knock out their nuclear potential. And we had to send the B-2s," Trump said, referring to the bat-winged US stealth bombers that carried out the bombings.
"I hope they're going to be more reasonable."
The remarks contrast with those by the US president on Friday, when he embraced potential regime change in Iran and lamented decades of failed talks.
Iran's Foreign Minister Abbas Araqchi met with the UN nuclear watchdog chief on Monday, saying in a post on X he was in Geneva to "achieve a fair and equitable deal".
"What is not on the table: submission before threats," Araqchi said.
The International Atomic Energy Agency has been calling on Iran for months to say what happened to its stockpile of 440kg of highly enriched uranium following Israeli-US strikes and let inspections fully resume, including in three key sites that were bombed in June last year.
Iran has repeatedly threatened to close the Strait of Hormuz in retaliation against any attack, which would choke a fifth of global oil flows and send crude prices sharply higher.
Iran held a military drill on Monday in the Strait of Hormuz, a vital international waterway and oil export route from Gulf Arab states, which have been appealing for diplomacy to end the dispute.
Despite Trump's comments about Iran seeking a deal, the talks face major potential stumbling blocks.
Washington has sought to expand the scope of talks to non-nuclear issues such as Iran's missile stockpile.
Tehran says it is only willing to discuss curbs on its nuclear program in exchange for sanctions relief and won't accept zero uranium enrichment. It says its missile capabilities are off the table.
