"Just spoke to [Minnesota] Governor Tim Walz and told him that the Military is with him all the way. Any difficulty and we will assume control but, when the looting starts, the shooting starts," he wrote.
Shortly afterwards, Twitter, which angered Mr Trump this week by tagging one of his tweets for the first time with a fact-check, said the post had violated rules about "glorifying violence".
However, the platform did not remove the post, saying that it "may be in the public’s interest" for it to remain visible.
"This Tweet violated the Twitter Rules about glorifying violence. However, Twitter has determined that it may be in the public’s interest for the Tweet to remain accessible," a disclaimer attached to tweet read.
Hours earlier, Mr Trump adopted a very different tone to the events.
"I feel very, very badly," he said about Mr Floyd's death.
"That's a very shocking sight."
Minneapolis police arrested Mr Floyd on Monday on suspicion of using a counterfeit banknote.
Police handcuffed him and held him to the ground, with bystander video showing an officer pressing his knee on Mr Floyd's neck.
The videos showed Mr Floyd saying that he couldn't breathe until he went silent and limp. He was later declared dead.
Minneapolis and neighbouring St Paul have been hit by widespread protests since.
Officials have assured angry residents that investigations into Mr Floyd's death were underway, and warned that violence would not be tolerated.
"We know there's a lot of anger. We know there's a lot of hurt," said St Paul Police Chief Todd Axtel.
"But we can't tolerate people using this as an opportunity to commit crimes," he said.
At the request of both cities, Minnesota Governor Tim Walz called up hundreds of National Guard troops and state police to help with security.
Two African American leaders of national stature, Jesse Jackson and Al Sharpton, arrived in Minneapolis and urged more protests.
"We told the governor you must call murder, a murder," Mr Jackson told an audience at the Greater Friendship Missionary Baptist Church.
"When you put ... your foot down somebody's neck until they can't breathe no longer, you murdered them," he said.
The UN human rights chief has condemned the death of Mr Floyd, demanding that US authorities take "serious action" to stop the killings of unarmed African Americans.
Additional reporting: AFP, Evan Young, Nick Baker.