In a post on Truth Social, Trump said he had been informed that "last night the Iranians shot down one of our highly sophisticated Apache Helicopters while patrolling over the Strait of Hormuz."
"The United States must, of necessity, respond to this attack," he added.
Pilots rescued, no injuries reported
According to US Central Command, the helicopter went down off the coast of Oman while conducting a patrol mission near the strategic waterway. Both crew members were rescued and are in stable condition.
Trump said the pilots were "fine" and suffered no injuries. Earlier reports had indicated that the cause of the incident was under investigation and it was unclear whether the aircraft had been brought down by hostile fire or mechanical failure.
First-ever sea rescue by US drone boat
The rescue operation marked a milestone for the US military. A US Navy autonomous surface vessel located the two pilots, who had spent around two hours in the water, before they were transferred to safety.
Captain Tim Hawkins of US Central Command described it as the first known rescue at sea conducted by an unmanned US military vessel.
Incident comes amid fragile Iran-Israel truce
The helicopter incident occurred just a day after Iran and Israel exchanged fire in what was described as the most serious violation yet of a fragile ceasefire between the two countries.
Trump's accusation against Iran threatens to further complicate efforts to stabilise the region and reopen the Strait of Hormuz, a crucial route for global oil shipments.
Trump expressed optimism about Iran deal
The latest escalation came despite Trump's earlier optimism about ongoing negotiations with Tehran.
"We have a good chance" of reaching an agreement in "two or three days," Trump had said, adding that Washington and Tehran were close to a "very, very good, strong, powerful deal."
He also argued that a military campaign against Iran would carry significant human costs.
"If we go and bomb — which we could do very easily if we want — and we spend another two or three weeks bombing, they'll have nothing left whatsoever. But you won't have the strait open for months," Trump said.
"If we do the bombing, a lot of people are going to be killed. Who wants to do that? I don't."
Apache helicopters key to US operations
AH-64 Apache attack helicopters have been extensively used by the US military during operations in and around the Gulf.
They have played a key role in enforcing pressure on Iran and countering drone threats in the region.
Trump has not yet outlined what form the promised US response to the alleged shoot-down could take.
However, his remarks are likely to raise concerns about a fresh escalation in tensions between Washington and Tehran.