Officials said the agent fell ill on Sunday and was receiving “much needed critical care” in Scotland.
A statement from the Secret Service said: “On 15 July a US Secret Service Special Agent suffered a cerebrovascular accident in Scotland, UK where he was travelling in support of Presidential Protection.
“The Secret Service asks for your prayers and continued respect for the agent and his family’s privacy.
“We are incredibly grateful for the emergency medical service providers and physicians in Scotland who are providing much needed critical care.”
The agency would not provide any further information on where the agent was being treated.
Police Scotland, the NHS and the Scottish Ambulance Service would not comment on the incident.
President Trump left Turnberry on Sunday, flying out of Prestwick Airport to Helsinki, Finland, ahead of talks with Russian president Vladimir Putin on Monday.