It convicted him of rape, murder, concealing the body and driving a motorcycle under the influence of methamphetamine. The court ruled that Ronnakorn raped the 27-year-old victim and then killed her by bludgeoning her head with a rock.
Ronnakorn was arrested in April after police discovered the body under rocks and leaves. Koh Si Chang is a popular tourist destination with a population of 5,000 people. The island is in Chonburi province, about 40 miles (65 kilometers) south of Bangkok.
Police investigator Somkid Kaensa said at the time of the arrest that a Thai tourist had notified police after seeing Ronnakorn's clothes covered in blood, and police then searched the area and discovered the tourist's body.
Somkid said a flower vendor who knew Ronnakorn also saw him in clothes covered in blood and asked him what had happened. The vendor identified Ronnakorn for police, who went to his home, where they found him and his bloodstained shirt, Somkid said.
In June last year, Thailand carried out its first execution in nine years. If it had reached 10 years without an execution it would have been reclassified as having abolished the death penalty in practice.