Jamila Henry, 21, was detained at a bus station in Ankara by Turkish police who suspected she was trying to get to Syria, the BBC understands.
She was deported and arrested by Metropolitan Police officers as she stepped off her flight from Istanbul.
Police said she was arrested on suspicion of preparing terrorist acts.
Ms Henry, who grew up in Streatham, south London, is being held in police custody, Scotland Yard said.
She is believed to have travelled to Turkey using the passport of her twin sister Jalila.
A British woman thought to have travelled to Turkey using her twin sister's passport has been arrested by counter-terror police at Luton Airport.