Officers said the arrest was Syria-related and the woman was being held at a south London police station.
The Metropolitan Police said two addresses in the north of the capital had been searched.
A Met statement said: "Officers from the Met's Counter Terrorism Command (SO15) have this evening, Thursday, 29 June, arrested a 21-year-old woman at Heathrow Airport as she disembarked a flight from Istanbul, Turkey.
"The woman, from north London, was arrested at 21:02hrs on suspicion of preparation of acts of terrorism under Section 5 of the Terrorism Act 2006."
In May, a 30-year-old man was arrested at Heathrow on suspicion of terror-related crime as he stepped off a plane from Istanbul
He was also held at a south London police station while officers searched a property in the east of the capital.
Woman held at Heathrow on suspicion of preparing terrorist attack
30 June 2017
Read: 1220
The woman, from north London, was arrested as she left a flight from Istanbul, Turkey, at just after 9pm on Thursday, AzVision.az reports citing the Independent.
-1780923278.jpg&h=190&w=280&zc=1&q=100)
