While the Senate is officially non-partisan, in reality the two main political camps are vying for control of the chamber in the absence of a functioning lower house following incomplete February polls.
The vote comes a day after tens of thousands of demonstrators took to Bangkok's streets to demand the resignation of Prime Minister Yingluck Shinawatra, who has resisted massive pressure to step down despite months of rallies and a slew of legal moves against her.
In contrast, the February 2 vote for lower house lawmakers was boycotted by the main opposition party and widely disrupted by anti-government protesters -- leading to its annulment earlier this month by the Constitutional Court.
With Thailand's political crisis lurching towards its sixth month, the Senate polls have taken on added importance.
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Experts say the elected portion -- a narrow majority of the 150-seat chamber -- could install many pro-government members to help bolster the administration in the face of looming legal challenges.
At a central Bangkok polling station a steady stream of voters arrived early today to cast their vote, according to AFP reporters.
"Elections are best for democracy. Whatever we do, we must have elections," said 65-year-old voter Amnuay Aransri.
Thailand has seen years of political conflict and rival street protests by opponents and supporters of Yingluck's brother, fugitive former premier Thaksin Shinawatra who was ousted in a coup in 2006.