Trencín is a city in western Slovakia of the central Váh River valley near the Czech border, around 120 km (75 mi) from Bratislava. It has a population of more than 55,000, which makes it the eighth largest municipality of the country and is the seat of the Trencín Region and the Trencín District. It has a medieval castle, Trencín Castle, on a rock above the city.

The site of Trencín has been inhabited since time immemorial. Trencín Castle, a typical medieval fortified castle is situated high on a rock above the city. Trencín is best known for a Roman inscription on the rock below the Trencín Castle dating from 179 AD, the era of the Marcomannic Wars, a series of wars between the Roman Empire and the Germanic Quadi and Marcomanni. It denotes the site as Laugaricio and for long time it was the most northern known evidence of the presence of Roman soldiers in central Europe (until the Roman fort by Mušov and marching camps by Olomouc and Hulín were found).

Trencín is one of the suggested locations for the capital of Samo's Empire in the 7th century. Wogastisburg (Vogast castle) was probably located somewhere on the Vah (Vogas) river and was also the site of a decisive battle between the Slavic and Frankish armies in 631.

It is plausible that Trencín Castle was founded during the Great Moravian era. In the beginning of the 11th century, the region was controlled by king Boleslaw I the Brave of Poland. In 1017, Stephen I of Hungary conquered the region which remained part of Hungary until 1918. By the end of the 11th century, the castle became the administrative centre of Trencsén county in the Kingdom of Hungary. As one of the few stone castles in the country it resisted the disastrous invasion of Mongols in 1241. In 1263 Trencín was in the possession of Jakab Cseszneky royal swordbearer, but in 1302 King Wenceslas I took it away from the Cseszneky brothers because they were supporting his rival Charles Robert, and donated it to Matthew III Csák. Between 1302 and 1321 the castle was the seat of the powerful magnate Matthew Csák who controlled most of what is now Slovakia. Challenging the authority of king Charles Robert, Matthew Csák maintained a large court and pursued his own foreign policy. The Treaty of Trentschin between Bohemia, Hungary, and Poland was signed in the city in 1335.

Trencín gained a number of privileges during the Middle Ages: In 1324 the inhabitants were freed from paying tolls and the city received free royal town privileges in 1412 from King Sigismund. However, during the following decades and centuries there were catastrophes and wars which lasted until the end of the 18th century. During the conflict between the Habsburgs and the supporters of the rival king, János Szapolyai, the town was captured in 1528 by imperial troops. In the 17th century the Ottomans were another threat from the south but they failed to conquer the city. The town then suffered from the Kuruc uprising against the Habsburgs and on 3 August 1708 the Battle of Trencín took place close to the city. Two years later a plague killed 1,600 inhabitants of the city. Finally, in 1790 the town, along with the castle, was burned down and the castle has been in ruins ever since.

In the 19th century Trencín flourished, as the railways to Zilina and Bratislava were built and many new enterprises were established, particularly in the textile, food and machine industries.

The town became the hub of the middle Povazie region.

In 1867 Trencín was downgraded from a "free royal town" to a "town with municipal government" and came under the direct control of the chief of Trencín county.

Trencín flourished again during the era of the first Czechoslovak republic and became the capital of the Trencín county again between 1940-1945 when the Slovak Republic was in existence.

Shortly after the Slovak National Uprising began, Trencín was occupied by Nazi Germany and it became the headquarters of the Sicherheitsdienst and the Gestapo.

Trencín was captured by the Romanian and Soviet troops on 10 April 1945.

Since 1990, the historical centre of the city has been largely restored and since 1996 it has been the seat of Trencín Region and Trencín District. The castle and its Roman inscription have attracted tourism since.