Police apprehended Jinda Pantharak, 56, at her home in a community near Nakhon Si Thammarat train station in tambon Klung of Muang district Thursday morning.
The arrest came after 22-year-old Wanchai Saengkhao told police he had bought two ya ba pills from her before committing the crime that shocked the nation.
Mrs Jinda, alias "Jae Da Train Station," allegedly confessed to selling the drug to Mr Wanchai twice - one pill in the first and two in the second time.
Police searched her home and found no illicit drugs, but did unearth a pen gun and six bullets.
Mrs Jindas claimed her son, who was arrested on drug charges last year, asked her to sell meth to his clients on his behalf.
Police will initially charge Mrs Jinda for illegal possession of firearms and are investigating further to identify more people involved in her drug network.