The unnamed man was arrested on Wednesday afternoon at Luton airport, north of London, on suspicion of being involved in the preparation of acts to commit terrorism.
The man is being held at a south London police station, Scotland Yard said.
Officers also searched a property in west London and the man, who was due to fly to Istanbul, remains in custody, according to a statement from the Metropolitan Police.
The same day the man was arrested in Luton, three other men, two aged 26 and one aged 27, were arrested in west London on suspicion of terrorism offences linked to Syria.
Hundreds of British Muslims have joined groups fighting President Bashar al-Assad's regime in Syria over the past three years, many travelling there by road from Turkey.