In a post on his own platform Truth Social, Trump claims that he made the decision at the request of Kristi Noem, minister for homeland security.
"I have given instructions to the Secretary of War Pete Hegseth to make all necessary forces available in order to protect the war-torn Portland", it says in the post.
Trump also claims that the immigration authority ICE's premises "are under attack from Antifa and other domestic terrorists".
Since the murder of the conservative activist Charlie Kirk earlier in September, the American president has stepped up his actions regarding confronting what he describes as "the radical left", which he accuses of being behind the country's problems with political violence.
Trump recently threatened Portland with sending the military there to curb crime. Similar threats have been directed at Chicago and Baltimore.
Donald Trump has previously deployed the National Guard in Los Angeles and Washington DC. Last week, he announced that Memphis is next in line.