Trump said on his Truth Social platform he was "designating" the movement as a terrorist organisation.
"I will also be strongly recommending that those funding ANTIFA be thoroughly investigated in accordance with the highest legal standards and practices," Trump wrote.
It was not clear what legal weight Trump's proclamation carried. Antifa, short for "anti-fascists", is a loosely organised ideological movement without a clear leadership structure or hierarchy, experts said.
A day after Utah prosecutors unveiled formal charges against the suspect in the assassination of Charlie Kirk, no evidence has emerged connecting 22-year-old Tyler Robinson with any outside group. Questions also remain about his precise motives.
Trump and senior officials have repeatedly blamed left-wing groups for creating an atmosphere of hostility towards conservatives before Kirk's assassination.
The White House is preparing an executive order on political violence and hate speech, a Trump administration official said earlier.
US Vice President JD Vance, in an interview with Fox News on Wednesday, local time, repeatedly blamed what he called left-wing political radicalisation for the assassination.
He said the White House was working hard to ensure that "funding networks for left-wing violence" are going to be treated like a terrorist organisation.
Critics say Trump is using Kirk's assassination as a pretext to crack down on political opponents.
Trump initially floated the idea of such a designation for Antifa in 2020 amid violent nationwide protests following the police killing of George Floyd in Minneapolis.
At the time, legal experts said such a step lacked a basis in law, would be hard to execute, and raised free-speech concerns, given that subscription to an ideology is not generally considered criminal.
The White House did not immediately respond to a request for further details.
With AP
