He was undergoing additional tests and would remain under observation for 24 hours as a precaution, broadcaster RTL reported, citing Luxembourg's ministry of state.
Bettel took part in a two-day EU summit in Brussels at the end of June, where participants included French President Emmanuel Macron, German Chancellor Angela Merkel, Italian Prime Minister Mario Draghi and other European leaders.
Luxembourg's state ministry could not immediately be reached for comment on Monday.