The man had been travelling with them, and was questioned twice by poice this week, the second time as a suspect on Wednesday, before being released.
"The DNA results of the semen do not match that of the friend," Police Sergeant Apichet Cherdguea told dpa.
"We have to start looking at the case again," he said, without indicating whose the DNA was.
The bodies of the tourists were found on the beach on Monday at the island resort of Koh Tao, 350 kilometres south of Bangkok.
The couple, both 24, suffered extensive injuries to the head and neck.
DPA