Trump also took aim at London mayor Sadiq Khan, tweeting, "At least 7 dead and 48 wounded in terror attack and Mayor of London says there is "no reason to be alarmed!"
And Trump used the series of tweets, which sparked criticism among some, to urge support for his executive order blocking travel to the United States from six Muslim-majority countries.
"We need to be smart, vigilant and tough," Trump tweeted. "We need the courts to give us back our rights. We need the Travel Ban as an extra level of safety!"
Seven people were killed, three attackers are dead and 48 victims were sent to local hospitals after a vehicle plowed into pedestrians on London Bridge. There were also stabbings at the nearby Borough Market.
The president's travel ban, which stemmed from his campaign promise to ban all Muslims from entering the U.S., is currently tied up in the courts. The Department of Justice recently asked the Supreme Court to weigh in.
Trump voiced support for London during the incident.
But he also retweeted a message from the Drudge Report about "fears of a new terror attack" roughly two hours before London authorities formally attributed the incidents to terror.
On Sunday morning, Trump tweeted, "We must stop being politically correct and get down to the business of security for our people. If we don't get smart it will only get worse."
He also used the attack to go after gun control advocates.
Another tweet read, "Do you notice we are not having a gun debate right now? That's because they used knives and a truck!"
Trump was briefed on the situation, according to the White House. According to federal law enforcement and national security officials, the National Security Council is monitoring the situation as are the appropriate elements of the FBI and Homeland Security.