Prime Minister Prayut Chan-o-cha has told Japanese Premier Shinzo Abe that the country will hold the general election in February.
Gen Prayut reaffirmed in talks with Mr Abe in Tokyo on Monday the government's roadmap to democracy, with the new poll to be held in February, according to the Government House statement.
The two met ahead of the Mekong-Japan Summit on Tuesday.
They also confirmed their cooperation to promote free trade as the Southeast Asian country has expressed its readiness to join a regional trade pact Japan has been involved in.
In a press briefing after their meeting in Tokyo, Mr Abe said Japan hailed Thailand's willingness to enter the 11-member Trans-Pacific Partnership agreement.
Japan has worked hard to conclude the TPP following the withdrawal of the United States last year under President Donald Trump and sought to expand the multilateral framework.
"Amid the spread of protectionism in the world, Japan agreed with Thailand, which leads the economic development in the Mekong region, on an early conclusion of the RCEP," Mr Abe also said, referring to the Regional Comprehensive Economic Partnership, another regional free trade pact involving 16 countries.
The 16 members -- Australia, China, India, Japan, New Zealand, and South Korea plus 10 members of the Association of Southeast Asian Nations -- are aiming for a broad agreement by the end of the year.
Mr Abe told Gen Prayut Japan will cooperate with Thailand to promote the Eastern Economic Corridor and train industrial personnel in the country.
In response, Gen Prayut said, "I appreciate Prime Minister Abe's push to encourage Japanese firms to invest in the Eastern Economic Corridor that is the centre of the Thai economy and would lead to the development of the Mekong region."
Mr Abe also expressed Tokyo's willingness to help Bangkok assume the rotating Asean chairmanship next year.