He is being held on suspicion of various terror offences, including one in Syria.
In a statement, the Metropolitan Police said: "Counter terrorism detectives have arrested a man at an airport in London in a pre-planned operation."
"Officers from the Met Police Counter Terrorism Command arrested the man, aged in his 20s, at Heathrow Airport at approximately 18.11hrs today, Wednesday, 3 February."
"The Section 5 arrest relates to suspected offending in Syria."
"The man - a British national - had arrived on a flight from Turkey."
"He was arrested under the Police and Criminal Evidence Act and taken into custody at a south-London police station, where he currently remains," said the force.