Also sentenced to life in prison was Lajos Gyorgy Kvalka, who denied a hand in the murder but admitted to helping David dispose of the body.
Both men are expected to lodge appeals within the coming month.
David was arrested in Surat Thani on December 4, a week after Mr Reisz's body was found. Mr Reisz – supposedly a friend and business partner of David – had been stabbed multiple times and bashed in the head, then dumped, wrapped in plastic in some woods not far from where he was murdered.
David was arrested in Bangkok and brought back to Phuket, where he initially admitted killing Mr Reisz.
On December 8, 2012, Kvalka was arrested. He denied taking part in the murder itself but agreed that he had helped David to wrap the body in plastic and dump it in the woods.
David and Kvalka will remain in custody until their appeal.
David also faces another trial, in Surat Thani, for the murder of Hungarian multi-millionaire Laszlo Csapai, 42, who had been missing since February 2011.
Csapai's skeleton was discovered buried under the floor of a villa on Koh Samui owned by David. His head had been bashed in.