Paetongtarn Shinawatra, a former prime minister, cast her vote at 10.30am at Polling Unit 15, inside a tent at Sports Ground 1 in the Kheha Thani Village 3 housing estate, Bangkok, accompanied by Khunying Potjaman Damapong and her elder sister, Pintongta Shinawatra Kunakornwong.
She appeared cheerful and greeted members of the public who had gathered at the site.
Paetongtarn was listed as voter number 464.
Upon arriving, Ms Paetongtarn queued to check the electoral roll and received ballot papers for the constituency MP and party-list MP votes before entering the booth to vote.
She then proceeded to cast her vote in the referendum.
After voting, supporters approached to take photos with Ms Paetongtarn and her family.
The group then returned home.
Later in the evening, Paetongtarn is scheduled to follow the vote count at the Pheu Thai Party.