The provision to make the Senate only partially elected was one of the most contentious clauses in the constitution, which was approved in Thailand’s first-ever referendum last year.
Analysts view the appointed seats as a way for the generals and Thailand’s royalist elite to keep their thumb on the fully elected lower house, which is dominated by allies of ousted premier Thaksin Shinawatra.
In seven hours of balloting, voters chose one senator to represent each of Thailand’s 76 provinces in the non-party election.
Seventy-four other senators have already been appointed by a commission set up last year by a new military-backed constitution.
8 killed in shooting
Eight people were killed after unknown gunmen stormed into a prominent doctor’s home in southern Thailand, but health officials ruled out any link to insurgent groups in nearby provinces. Dr Suthathip Thammachart was killed along with seven of his friends at his residence.