Senator Pornthip has stepped down effective as of today, fulfilling her pledge to serve for only two years.
She voiced hope that the election of her successor would proceed in a smooth manner.
She also said elected and appointed senators should have an equal chance of filling the vacant position.
Senate Speaker Nikom Wairatpanij said each candidate would be given five minutes to deliver a vision statement before the vote.
The vote will likely take place in the afternoon, as the morning session is scheduled for the naming of board members tasked with monitoring the performance of the National Broadcasting and Telecommunication Commission.