The woman was arrested in Thalang by one group of officers as a second group were on their way to Koh Yao, off Phuket's east coast, to arrest a man.
Police believe the woman ordered the killing and the man found the shooter who carried out the murder.
Three men have already been arrested. Sawit Jongrak, 28, allegedly fired at Khun Supattra, hitting her in the head. Aroon Patan, 28, and Sanawg Punerng, 26, are accused of being accomplices.
The three men were in a pickup stopped by police at Tachatchai checkpoint, on the main road leading off Phuket, minutes after the shooting. Khun Supattra, 55, died soon after in hospital.
Investigators have yet to determine the motive for the shooting, for which the men were allegedly paid a 40,000 baht deposit on a fee of 80,000 baht.
A British man is believed to have been friendly with both women.