He was one of five men arrested on Thursday on suspicion of supporting proscribed organisations.
Four have been bailed but police have been given more time to question the 19-year-old.
Two of the arrested men have been named as Rofi Islam and Sajid Idris.
Officers from the North East Counter Terrorism Unit and the Wales Extremism and Counter Terrorism Unit (WECTU) executed six search warrants on Thursday.
WECTU said the investigation "does not concern any immediate threat to public safety".
Thursday's arrests were not linked to two men charged with offences on Wednesday.
Police said the arrests were linked to the Grangetown area of Cardiff and were part of a wider counter-terrorism investigation in Wales, but are not linked to brothers Aseel and Nasser Muthana who went to fight with IS in Syria.
The raids in Grangetown were within a few streets of each other. Witnesses described how a number of police vehicles descended on the area.
South Wales Police Assistant Chief Constable Nikki Holland denied Cardiff was a hotbed of terrorism.
She said the Muthana brothers and Reyaad Khan had become poster boys in the UK for IS but said police were determined to tackle radicalisation.
A 19-year-old man being held following anti-terror raids in Cardiff and Barry has been arrested on suspicion of preparing or assisting in the preparation of a terrorist act.