"Our Great Larry Kudlow, who has been working so hard on trade and the economy, has just suffered a heart attack," Trump tweeted.
Kudlow has been taken to Walter Reed Medical Center, the president said.
The announcement came less than a half hour before Trump's historic meeting with North Korean leader Kim Jong Un.
Kudlow’s friend, economist Steven Moore, told ABC News, “He’s going to be fine.”
“I expect him to make a full recovery,” Moore said. “He’ll be back on the job.”
Kudlow, 70, made news Sunday after commenting on CNN that Canadian Prime Minister Justin Trudeau engaged in "betrayal" and "stabbed us in the back" at a press conference after the G-7 meeting over the weekend.
In response to the U.S. decision to apply tariffs on some Canadian imports, Trudeau said "retaliatory measures" would be forthcoming.
The prime minister said he told Trump that Canada would "move forward with retaliatory measures on July 1, applying equivalent tariffs to the ones that the Americans have unjustly applied to us."
Trump chose Kudlow, a former Reagan official and conservative economic commentator, to lead the National Economic Council in March following the resignation of Gary Cohn over a profound disagreement on tariffs.
Like Cohn, Kudlow was an opponent of Trump's tariff policy.
In a March 3 column, Kudlow wrote that tariff hikes are in effect tax hikes for consumers, adding that the president should "examine the historical record on tariffs, because they have almost never worked as intended and almost always deliver an unhappy ending."
The day before announcing that Kudlow would take over for Cohn, Trump said that Kudlow had "come around" on the tariff issue.
Since Trump announced Kudlow's heart attack, several political figures have tweeted support for the adviser and his family.
Press secretary Sarah Sanders said late Monday in a statement that Kudlow "experienced what his doctors say, was a very mild heart attack. Larry is currently in good condition at Walter Reed National Military Medical Center and his doctors expect he will make a full and speedy recovery. The President and his Administration send their thoughts and prayers to Larry and his family."