"Quite an evening in DC Secret Service and Law Enforcement did a fantastic job. They acted quickly and bravely. The shooter has been apprehended," Trump wrote on his Truth Social platform, Anadolu Agency reported.
His remarks came after the US president and first lady, Melania Trump, were evacuated from the annual event at the Washington Hilton Hotel after a security-related incident.
In a separate post, Trump said the first lady, Vice President JD Vance and all Cabinet members, are "in perfect condition."
"We will be speaking to you in a half an hour. I have spoken with all the representatives in charge of the event, and we will be rescheduling within 30 days," he added.
The Secret Service said one individual is in custody. "The condition of those involved is not yet known, and law enforcement is actively assessing the situation," it said.
Later, addressing reporters, Trump said an officer was shot. "He's in great shape."
Media reports said the suspect was identified as Cole Thomas Allen, 31.
"I guess he lives in California, and he's a sick person, a very sick person," Trump said, adding the shooter was a “lone wolf.”
'Defendant will be arraigned on Monday in federal district court'
In a separate news conference, DC Mayor Muriel Bowser said the gunman rushed a Secret Service checkpoint in the lobby of the hotel.
"Secret Service agents stopped the suspect. An agent was injured and transported to a local hospital, where the agent is receiving care," said Bowser.
The suspect was also transported to a hospital where he is being evaluated, she added.
"We have no reason to believe at this time that anyone else was involved," said the mayor.
Jeff Carroll, interim chief of police for the Metropolitan Police Department, said the suspect was armed with a shotgun, a handgun and multiple knives.
According to preliminary information, Carroll said he was a guest at the hotel.
The officer said he cannot give a motivation for the suspect’s actions "at this point."
US Attorney for the District of Columbia Jeanine Pirro said the suspect is being charged with two counts.
"The defendant will be arraigned on Monday in federal district court," Pirro added.