The 21-year-old, from St Austell, was arrested on suspicion of robbery on Thursday and remains in police custody.
A string of robberies and attempted robberies took place at shops in Reawla, Cubert, Scorrier, Porthtowan and Lanner over the last three weeks.
Police said isolated premises were targeted and a knife had been used to threaten staff.
A man is being questioned by police over a series of armed robberies which took place across Cornwall.