The Criminal Court has found Watana Pumret, a retired engineer, guilty of planting the bomb that exploded at Phramongkutklao Hospital in May. They sentenced him to another 27 years in prison.
The former electricity generating authority (Egat) employee was sentenced to four years in prison in September for illegally making and possessing explosives and illegally possessing black powder.
The 62-year-old confessed to masterminding the explosion that wounded 21 people and caused damage worth over one million baht at the military-run hospital.
In its ruling on Wednesday, the court convicted him on four charges - attempted murder, planting a bomb, causing serious damage and possessing explosives in public.
He was initially sentenced to life but the penalties were commuted to a total of 27 years in prison because he cooperated, local media reported.
The court heard that Watana drove his car to the Egat office in Nonthaburi. He then changed his clothes and travelled to the hospital by bus.
He had a plastic bag containing a vase which concealed a bomb. At the hospital, he went to the waiting room and then left, leaving the bag behind.
Watana claims he acted alone and did not mean to injure anyone. He said he despised the military-led government and intended the bomb as a "statement" against them.
The explosion on May 22 coincided with the third anniversary of the 2014 coup against the former Yingluck Shinawatra administration.
Watana is also suspected of involvement in other bombings between 2007 and 2017, mainly in Bangkok.
