The first was arrested at around 09.20 GMT as she tried to board a flight to Istanbul in Turkey from Heathrow.
She was seized on suspicion of being involved in the commission, preparation and instigation of acts of terrorism. A "large quantity" of money was seized.
A second woman, aged 27, was held on suspicion of the same offence in north-west London and taken into custody.
Both women are now being questioned.
Scotland Yard said the arrests were intelligence-led rather than in response to an immediate threat or risk.
Two addresses in north and north-west London are now being searched.
Two women aged 26 and 27 have been arrested on suspicion of terrorism offences, the Metropolitan Police says.