"I also want to send best wishes to a very good friend of mine, a friend to our nation, Prime Minister Boris Johnson. We were very saddened to hear that he was taken into intensive care this afternoon," the president told a White House news conference.
"Americans are all praying for his recovery. He's been a really good friend. He's been really something very special - strong, resolute, doesn't quit, doesn't give up," Trump said.
Johnson, 55, was admitted to a hospital on Sunday night and had been undergoing tests after suffering persistent coronavirus symptoms, including a high temperature, for more than 10 days.
He was moved to an intensive care unit on Monday.
Trump, who spoke earlier on Monday with executives from pharmaceutical and biotechnology companies, said he had asked two of the companies to get in touch with the British government to offer their services.
"They've really advanced therapeutics ... and they have arrived in London already. Their London office has whatever they need and we'll see if we can be of help. We've contacted all of Boris' doctors and we'll see what is going to take place. But they are ready to go," Trump said.
"But when you get brought into intensive care, it gets very, very serious with this particular disease."