The four were arrested at their homes in the southwest of Birmingham, Britain's second city, in central England, West Midlands Police said.
A 45-year-old man was arrested on suspicion of attending a terror training camp and facilitating terror abroad.
A 44-year-old woman and her son aged 20 were held on suspicion of facilitating terror abroad, as was a 36-year-old man at a separate address.
The three homes were being searched by officers from the West Midlands Counter Terrorism Unit, while vehicles and electronic equipment were being removed for forensic analysis.
The suspects, who were not named, were being held at a police station in the Birmingham area.
