Tiruchirappalli (Tamil pronunciation: [ˈt̪iɾɯtːʃiɾapːaɭːi] ^ⓘ), also known as Trichy, is a major tier II city in the Indian state of Tamil Nadu and the administrative headquarters of Tiruchirappalli district. The city is credited with being the best livable and the cleanest city of Tamil Nadu, as well as the fifth safest city for women in India. It is the fourth largest urban agglomeration in the state. Located 322 kilometres (200 mi) south of Chennai and 374 kilometres (232 mi) north of Kanyakumari, Tiruchirappalli sits almost at the geographic centre of Tamil Nadu. The Cauvery Delta begins 16 kilometres (9.9 mi) west of the city where the Kaveri river splits into two, forming the island of Srirangam which is now incorporated into the Tiruchirappalli City Municipal Corporation. The city occupies an area of 167.23 square kilometres (64.57 sq mi) and had a population of 916,857 in 2011.

Tiruchirappalli's recorded history begins under Chola rule in the 3rd century BC. The city has also been ruled by the Pallavas, Pandyas, Vijayanagar Empire, Nayak Dynasty, the Carnatic state and the British. The most prominent historical monuments in Tiruchirappalli include the Rockfort at Teppakulam, the Ranganathaswamy temple at Srirangam dedicated to the reclining form of Hindu God Vishnu, and is also the largest functioning temple in the world, and the Jambukeswarar temple at Thiruvanaikaval, which is also the largest temple for the Hindu God Shiva in the world. The archaeologically important town of Uraiyur, capital of the Early Cholas, is now a neighbourhood in Tiruchirappalli. The city played a critical role in the Carnatic Wars (1746–1763) between the British and the French East India companies.

The city is an important educational centre in the state of Tamil Nadu, and houses nationally recognized institutions such as National Institute of Technology - Tiruchirapalli (NIT-T), Indian Institute of Management (IIM), Indian Institute of Information Technology (IIIT), Tamil Nadu National Law University (NLU), Government Medical College. Industrial units such as Bharat Heavy Electricals Limited (BHEL), Golden Rock Railway Workshop, Ordnance Factory Tiruchirappalli (OFT) and High Energy Projectile Factory (HEPF) have their factories in the city. The presence of a large number of energy equipment manufacturing units in and around the city has earned it the title of "Energy Equipment and Fabrication Capital of India". It is one of the few towns and cities in List of AMRUT Smart cities in Tamil Nadu selected for AMRUT Schemes from central government and the developmental activities are taken care by government of Tamil Nadu. Tiruchirappalli is internationally known for a brand of cheroot known as the Trichinopoly cigar, which was exported in large quantities to the United Kingdom during the 19th century.

A major road and railway hub in the state, the city is served by the Tiruchirappalli International Airport (TRZ) which operates direct flights to the Middle East (Dubai, Saudi Arabia) and Southeast Asia (Singapore, Malaysia).

Historically, Tiruchirappalli was commonly referred to in English as "Trichinopoly". The shortened forms "Trichy" or "Tiruchi" are used in everyday speech and the full name Tiruchirapalli appears in official use by government and quasi-government offices but seldom used by the general public.

A Hindu historical account describes Tiruchirapalli's name as being derived from "Thiru-Sriranga-Palli", as the place where the reclining lord Ranganatha reposed. . Another account from Hindu mythology describes Tiruchirappalli as deriving its name from the three-headed demon Trishira, who meditated on the Hindu god Shiva near the present-day city to obtain favours from the god. An alternative derivation, albeit not universally accepted, is that the source of the city's name is the Sanskrit word "Trishirapuram"—Trishira, meaning "three-headed", and palli or puram meaning "city"..

According to the late scholar C. P. Brown, Tiruchirappalli might be a derivative of the word Chiruta-palli (lit. 'little town'). Orientalists Henry Yule and Arthur Coke Burnell have speculated that the name may derive from a rock inscription carved in the 16th century in which Tiruchirappalli is written as Tiru-ssila-palli, meaning "holy-rock-town" in Tamil. Other scholars have suggested that the name Tiruchirappalli is a rewording of Tiru-chinna-palli, meaning "holy little town". The Madras Glossary gives the root as Tiruććināppalli or the "holy (tiru) village (palli) of the shina (Cissampelos pareira) plant".

Tiruchirappalli is one of the oldest inhabited cities in Tamil Nadu; its earliest settlements date back to the Sangam period. Uraiyur, the capital of the Early Cholas for 600 years from the 3rd century BC onwards, is a neighbourhood in the present-day Tiruchirappalli. The city is referred to as Orthoura by the historian Ptolemy in his 2nd-century work Geography. The world's oldest surviving dam, the Kallanai (Lower Anaicut) about 18 kilometres (11 mi) from Uraiyur, was built across the Kaveri River by Karikala Chola in the 2nd century AD.