The 27-year-old is a UK national.
She had just arrived on a flight from the Ethiopian capital of Addis Ababa when she was arrested on Friday.
Police said she had been taken to a south London police station, where she remains in custody.
In a statement, police said: “Officers from the Met’s counter terrorism command have arrested a 27-year-old woman at Heathrow Airport.
“The woman, a UK national, arrived on a flight from Addis Ababa and was arrested on suspicion of preparation of terrorist acts under section 5 of the Terrorist Act 2006.