The 30-year-old was held in Somerset at around 12.20pm on Wednesday and Scotland Yard said a house in South Devon and a wooded area were being searched.
He was arrested on suspicion of preparation for acts of terrorism in a pre-planned swoop as part of an investigation into Northern Ireland-related terrorism.
Police in Northern Ireland are also carrying out searches as part of the investigation.
Scotland Yard said: “A 30-year-old man has been arrested in Somerset on suspicion of terrorism offences.
“He was arrested at 12.20pm on Wednesday August 24 by officers from the Metropolitan Police Service’s Counter Terrorism Command, supported by colleagues from Avon and Somerset and Devon and Cornwall Police, on suspicion of being involved in the preparation of acts of terrorism under Section 5 of the Terrorism Act.
“Today’s arrest was pre-planned and intelligence-led as part of an investigation into Northern Ireland-related terrorism. The public can be reassured that there is no intelligence to suggest an immediate threat to our communities.”
No armed police were involved in the arrest.
The threat to England, Wales and Scotland from Northern Ireland-related terrorism is currently rated as substantial, while the threat to Northern Ireland itself is ranked as severe.