Thammasat University (TU; Thai: มธ.; Thai: มหาวิทยาลัยธรรมศาสตร์, RTGS: Mahawitthayalai Thammasat) is a public research university in Thailand with campuses in the Tha Phra Chan area of Bangkok, Rangsit, Pattaya and Lampang. As of 2024^[update], Thammasat University has over 39,000 students enrolled in 33 faculties, colleges, institutes and 2,000 academic staff.

Thammasat is Thailand's second oldest university. Officially established to be the national university of Thailand on 27 June 1934, it was named by its founder, Pridi Banomyong, the University of Moral and Political Sciences (Thai: มหาวิทยาลัยวิชาธรรมศาสตร์และการเมือง; RTGS: Mahawitthayalai Wicha Thammasat Lae Kanmueang). It began as an open university, with 7,094 students studying law and politics in its first year.

In 1960, the university ended its free-entry policy and became the first in Thailand to require passing national entrance examinations for admission. Thammasat today offers more than 240 academic programmes in 33 different faculties and colleges on four campuses. Over the 80 years since its foundation, Thammasat University has evolved from an open university for law and politics to an international university offering all levels of academic degrees in many fields and disciplines. It has graduated more than 300,000 undergraduate and graduate students. The university's alumni have included some of Thailand's prime ministers, leading politicians, governmental figures, Bank of Thailand governors, and jurists.

Tha Phra Chan Campus, the original campus of the university, is in Phra Nakhon, Bangkok. The campus is in close proximity to many tourist destinations and was the site of the 14 October 1973 uprising and the 6 October 1976 massacre. Rangsit campus, where most undergraduate programmes are concentrated, is in Khlong Luang, Pathum Thani. Thammasat has smaller regional campuses in Lampang and Pattaya.

The name "Thammasat" derives from the Buddhist-term Dhammasattha, meaning "study of law".

Thammasat University began in 1934 as the University of Law and Political Sciences. This was two years after the so-called Siamese revolution of 1932 and eighteen years after the founding of Chulalongkorn University by transforming the law school of Prince Raphi Phatthanasak Krommaluang Ratcha Buri Direk Rit, which dated back to 1907. Thammasat University was the brainchild of Pridi Banomyong, the father of Thailand's democracy and the minister of interior, who drafted the "University of Law and Political Science Act 1934". The university was inaugurated on 27 June 1934, and Pridi served as the university's first chancellor.

The university is based on the sixth principle of the People's Party. The first announcement of Khana Ratsadon stated the government "must provide the people with full education" because people "lack education, which is reserved for royals". The desire of students at the school of law to be upgraded to a university rather than simply a department at Chulalongkorn University also helped Thammasat University become the successor of the law school. The property and faculty of the law school were transferred to University of Law and Political Science, and the old law school building was the first Thammasat site. The university moved to Tha Phra Chan campus the following year.

When the university opened, 7,094 people applied for admission. At that time, Chulalongkorn University was graduating only 68 students a year. Thammasat initially offered a bachelor's degree with an emphasis on legal studies and previously banned economics and political science, plus a bachelor's degree equivalent diploma in accountancy. Master's degree courses soon followed in law, political science, and economics, and doctoral degree courses in law, political science, economics, and diplomacy.