The Criminal Court has found Watana Pumret, a retired engineer, guilty of planting the bomb that exploded at Phramongkutklao Hospital in May and sentenced him to another 27 years in prison.
The former electricity generating authority (Egat) employee was earlier sentenced to four years in prison after the court convicted him in September on two charges - illegally making and possessing explosives and illegally possessing black powder.
The 62-year-old confessed to masterminding the explosion that wounded 21 people, including retired officers, 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 for planting a bomb and attempted murder, three years for assembling a bomb and one year for illegal possession of explosives, local media reported.
The penalties were commuted to a total of 27 years in prison because he cooperated.
Watana was also fined 500 baht, reduced from 1,000 baht, for carrying the bomb in a public place.
The court heard that Watana drove his car to the Egat office in Nonthaburi and parked. He then changed his clothes and travelled to the hospital by bus, carrying a plastic bag.
In the bag was a flower vase with a bomb hidden inside it. At the hospital, he went to the dispensary waiting room, known as the Wongsuwon room, and then departed, leaving the bag behind.
The court viewed security camera footage showing a man matching Watana's physical appearance inside the room, and heard evidence of his secret bomb making.
Watana earlier said he committed the bombing alone, and because he despised the military-led government. However, he did not mean to injure people. He intended the bomb only as a "statement" against the government.
The explosion on May 22 coincided with the third anniversary of the 2014 coup against the former Yingluck Shinawatra administration.
Police also suspected Watana was involved in other bombings between 2007 and 2017, mainly in Bangkok.