The small Pacific island is a US territory about 2,200 miles (3,500 km) south east of the Korean peninsula.
North Korea has said it would make an “enveloping fire” around the militarily strategic island.
It has been threatened by Pyongyang before, but tensions are heightened after reports that the isolated nation has been able to make a nuclear warhead to fit on an intercontinental ballistic missile (ICBM) that could theoretically reach the US.
Guam’s Governor Eddie Baza Calvo posted a video on Facebook of himself taking the approximately three minute call from Mr Trump when he made the comments.
“We are with you 1,000 per cent, you are safe,” Mr Trump said.
Though the likelihood of an attack is low according to experts with whom The Independent has spoken, Guam has still issued guidelines for it’s roughly 163,000 residents on what to do in case of a nuclear attack.
Japan has also assured its support for the island should an attack be imminent.
According to Pyongyang’s public statements, the missile would reach the waters off the coast of Guam in approximately 17 minutes.
The island is home to American nuclear submarines but also the Terminal High Altitude Area Defence (Thaad) system that could used to thwart an attack.
Mr Trump went on to say that he has been watching news reports about Guam and that the “beautiful” island has “become extremely famous. All over the world they are talking about Guam, and they are talking about you.”
Tourism and money generated by the two US military bases and its occupants are the main sources of revenue for the territory. Mr Calvo agreed with the President that Guam is “paradise”.
Many in Washington and the diplomatic community have criticised Mr Trump for his “fire and fury” comments to North Korea, saying that it does regional peace no good to mimic Kim Jong-un‘s bombastic rhetoric.
“So with all the criticism going on over there, from a guy that’s being targeted, we need a president like you, so I’m just so thankful, and I’m glad you’re holding the helm, sir,” Mr Calvo said on the call, playing to the President’s ego.
Naturally, Mr Trump agreed: “They should have had me in years ago … frankly you could have said that for the last three presidents.”