The Constitution of the Kingdom of Thailand (รัฐธรรมนูญแห่งราชอาณาจักรไทย; ) provides the basis for the rule of law in Thailand. Since the abolition of the absolute monarchy in 1932, Thailand has had 20 charters or constitutions (as of 2015), an average of one roughly every four years. Many changes followed military coups, reflecting the high degree of political instability in the country. After each successful coup, military regimes abrogated the existing constitution, generally without public consultation.
The 1997 constitution of Thailand, often called the "people's constitution", was considered a landmark in terms of the degree of public participation involved in its drafting as well as the democratic nature of its articles. It stipulated an elected bicameral legislature, and many human rights were explicitly acknowledged for the first time. Many of these reforms disappeared in the military coup of 2006.
The current constitution was adopted in 2017. The 105-page, 279-article proposed constitution was approved by 61.4 percent of Thai voters with 59.4 percent of the public participating. It allows the National Council for Peace and Order (NCPO) to appoint an eight to ten person panel who will choose Senators, and includes six seats reserved for the heads of the Royal Thai Army, Navy, Air Force, and Police, plus the military's supreme commander, and defense permanent secretary. The bicameral Parliament could also select a candidate as Prime Minister who is not one of its members or even a politician. Critics suggest it effectively allows the military to control the government whatever the outcome of subsequent elections.
The Rattanakosin Kingdom and the four traditionally counted preceding kingdoms, collectively called Siam, had an uncodified constitution until 1932. In the preamble to the Penal Code promulgated 1 April 1908, which came into effect on 21 September, King Chulalongkorn (Rama V) stated: "In the ancient times the monarchs of the Siamese nation governed their people with laws which were originally derived from the Dhamasustra of Manu, which was then the prevailing law among the inhabitants of India and the neighbouring countries.