Amnesty International said the bomb attacks on the Pattani Big C on Tuesday showed a "callous disregard for human life".
The long-time human rights group called on the government to carry out an independent investigation to bring the attackers to justice - but without the death penalty.
The full statement by AI:
Responding to two bombs that struck a shopping centre in the southern Thai city of Pattani, Amnesty International's Director for Southeast Asia and the Pacific, Champa Patel, said:
"The attack on a shopping centre in Pattani is a horrific and deliberate attack on civilians, and shows a callous disregard for human life.
"Thailand's authorities must immediately order an independent and effective investigation. They have a duty to bring the perpetrators to justice through fair trials and in line with international standards and without imposing the death penalty.
"All steps taken by the authorities to stop and prevent such attacks must respect Thailand's obligations under international human rights law."