Mrs May is understood to have raised the issue during a "cordial" telephone call.
The whole event was a little surreal: the President had gone outside to watch an 11-year-old called Frankie Giaccio mow the lawn (Frankie had written to the White House asking if he could) and ended up talking global affairs.
He also suggested potential attackers were "in the sights of Scotland Yard".
Donald Trump tweeted: 'Another attack in London by a loser terrorist.These are sick and demented people who were in the sights of Scotland Yard.
An improvised explosive device (IED) exploded on a packed underground train at a London station today, injuring at least 22 people in the fifth terrorist attack this year, which May said was "clearly meant to cause significant harm".
"I spoke with a wonderful woman, British Prime Minister Theresa May, and relayed America's deepest sympathy, as well as our absolute commitment to eradicating the terrorists from our planet".
It was at 6.42am in Washington that Donald Trump tweeted himself into another diplomatic row with the UK.
May said it's not helpful when anyone speculates during an ongoing investigation..
No group immediately claimed responsibility for Friday's attack and police have no suspects in custody.
Trump says he is preparing to call May.
A former adviser to Theresa May, Nick Timothy, responded to Trump's tweets nearly instantly, questioning whether it was based on actual intelligence.
Israeli officials, ministers and dignitaries were also quick to convey their support on social media. That's because they used knives and a truck!
The bomber's identity and details of the probe leaked to United States media before British officials felt ready to disclose them.
The White House did not immediately respond to questions about Trump's assertions.
His comments drew a rare public statement from GCHQ denouncing the allegation as "utterly ridiculous".
Sadiq Khan, the Muslim mayor of London, said that his city "utterly condemns the disgusting individuals who attempt to use terror to harm us and destroy our way of life". "As London has proven again and again, we will never be intimidated or defeated by terrorism", he said.
"I've simply been too busy this morning to look at my Twitter", he told LBC Radio.
"I'm not going to go there ... my priority is making sure we are doing what we can to make London safe". I want the facts. "That's quite clear what he thinks about that".
This is not the first time Trump has rapidly issued tweets in the wake of a terror attack in the United Kingdom.