Officers detained the 33-year-old on Thursday afternoon and he is being held at a central London police station.
The Met Police said the warrant for further detention lasts to Thursday.
He is being questioned on suspicion of the commission, preparation and instigation of terrorist acts under section 41 of the Terrorism Act 2000.
Police were granted the further extension at a magistrates' court on Saturday.
Following the arrest in Cardiff, police searched two houses.
Officers said the arrest was a result of an investigation by the MPS Counter Terrorism Command supported by the Wales Extremism and Counter Terrorism Unit.
There was police activity in the Grangetown and Riverside areas of Cardiff on Thursday afternoon but Metropolitan Police refused to confirm if it was linked to the arrest.
The BBC understands the arrest is related to alleged Islamist terrorism.
Police have been granted more time to question a man arrested on suspicion of terrorism offences in Cardiff, the Met Police said.