“Our Great Larry Kudlow, who has been working so hard on trade and the economy, has just suffered a heart attack”, Mr Trump said on Twitter.
Mr Kudlow was receiving treatment at Walter Reed Medical Center in Bethesda, Maryland, Mr Trump said.
White House press secretary Sarah Huckabee Sanders subsequently told reporters that Mr Kudlow was “in good condition”, saying the heart attack was a mild one.
The timing of Mr Kudlow’s apparent medical emergency was notable for multiple reasons.
Over the weekend, Mr Kudlow amplified Mr Trump’s aggressive views on trade by saying Canadian prime minister Justin Trudeau had “stabbed America in the back” for pushing back on US trade demands after Mr Trump left a G-7 conference.
And Mr Trump wrote his message of support from Singapore, just minutes before he was expected to meet North Korean leader Kim Jong-un for a high-stakes and historically fraught summit.
Mr Kudlow became director of the National Economic Council in April, replacing Gary Cohn as the Trump administration swung towards a more protectionist trade strategy.
Acting on Mr Trump’s longstanding contention that other countries have taken advantage of America with one-sided trade deals, the administration has unveiled a series of tariffs intended to counteract what Mr Trump calls unfair trade imbalances.
The president's push for steep steel and aluminium tariffs was said to have divided Mr Trump’s economics team, with Mr Cohn among those who unsuccessfully urged the president to reconsider and touted the benefits of free trade.