Michelle Elizabeth Smith, aged 59, was stabbed in the chest as two men on a motorcycle tried to rob her last Wednesday near her hotel in Phuket town.
One of the suspects was detained near Bangkok on Monday and the other in the southern province of Chumphon, Police Major General Chanasit Vatanavarangoon said by telephone from Phuket, without disclosing their identities.
"They were charged with robbery and murder and they both confessed," he said.
The pair, who both fled to the mainland after the killing, were filmed by a security camera during the attack.
The victim was part of a group of tour operators visiting the island. One of her travel companions was hospitalised after suffering a cut to her arm.
The death was another blow to Thailand's tourist-friendly image, coming less than a week after two Canadian sisters were found dead in their hotel room on nearby Phi Phi island, showing signs of an extreme toxic reaction