The downing of the drone -- which Iran insists violated its airspace, a claim Washington denies -- has seen tensions between the countries spike after a series of attacks on tankers the US has blamed on Tehran.
Under pressure to respond to the high-stakes incident near the strategic Strait of Hormuz, Trump said the US was "cocked & loaded to retaliate" on Thursday night by hitting "3 different sites," but scrapped the strikes just 10 minutes before they were carried out.
"I asked, how many will die. 150 people, sir, was the answer from a General," the president tweeted, saying he concluded it would not have been "proportionate to shooting down an unmanned drone."
"I am in no hurry, our Military is rebuilt, new, and ready to go," he wrote.
The US president had struck a combative tone in his initial public comments Thursday about Iran shooting down the Global Hawk surveillance aircraft, but as the pre-dawn incident whipped up fears of open conflict, Trump moved to dial back tensions.
Iran vowed Friday to defend its borders after downing the drone, with the commander of the aerospace arm of its elite Revolutionary Guards saying the aircraft was warned twice before it was engaged over the Gulf of Oman.
And it denied a report that Trump had warned it via Oman of an impending attack unless it was willing to negotiate.