"National Security Adviser Robert O'Brien tested positive for COVID-19. He has mild symptoms and has been self-isolating and working from a secure location off site. There is no risk of exposure to the President or the Vice President. The work of the National Security Council continues uninterrupted," the White House said in a statement Monday morning.
"I haven’t seen him lately. I heard he -- he tested. Yeah. I have not seen him. I’m calling him later," Trump told reporters.
Trump said he didn't know when O'Brien first tested positive.
It comes as new coronavirus cases and hospitalizations surge across the American South and West.
It's unclear how O'Brien, 54, was exposed to the coronavirus or how much in-person contact he's had recently with Trump. Their last public appearance together was on July 10 during a visit to the U.S. Southern Command in Miami.
Administration officials close to the president are regularly tested for COVID-19, and Trump receives rapid coronavirus tests on a daily basis, according to the White House.
O'Brien was last seen at the White House on Thursday, multiple sources told ABC News. However, it's still unclear as to when he was last with the president. It's also not clear when O'Brien first tested positive.
When someone who works in the White House complex tests positive for COVID-19, the White House management office usually sends out an email to all staff. In this case, many White House staffers were caught off guard -- learning from news reports -- that a senior adviser who works in the West Wing had tested positive, according to sources.
O'Brien's National Security Council office in the White House is near the Oval Office and Vice President Mike Pence's West Wing office.
He is the highest-ranking official close to the president to contract the virus -- but he's not the first person connected to the Trump administration to test positive for the coronavirus.
Kimberly Guilfoyle, who is dating the president's eldest son, Donald Trump Jr., tested positive for the coronavirus while in South Dakota ahead of Trump's Mount Rushmore event in early July.
Pence's press secretary, Katie Miller, wife to the president's senior adviser Stephen Miller, also tested positive in May.
Also in May, a U.S. military service member who works at the White House campus and serves as a valet to the president -- bringing him his lunch among other service items -- also tested positive for COVID-19.
Dozens of Secret Service officers and agents who were on site for Trump's June rally in Tulsa were ordered to self-quarantine last week after two of their colleagues tested positive.
At least eight of the president's campaign staffers have also tested positive -- two of them getting positive results after attending the Tulsa rally, the Trump campaign confirmed to ABC News.
There have been 16.2 million cases of the coronavirus worldwide since the pandemic began earlier this year, with 146,935 deaths in the U.S., according to data from Johns Hopkins University.
ABC News' Katherine Faulders and Will Steakin contributed to this report.
What to know about the coronavirus:
Tune into ABC at 1 p.m. ET and ABC News Live at 4 p.m. ET every weekday for special coverage of the novel coronavirus with the full ABC News team, including the latest news, context and analysis.