Třinec (Czech: [ˈtr̝̊ɪnɛts] ^ⓘ; Polish: Trzyniec [ˈtʂɨɲɛt͡s] ^ⓘ; Silesian: Trzińec, German: Trzynietz) is a city in Frýdek-Místek District in the Moravian-Silesian Region of the Czech Republic. It has about 34,000 inhabitants, making it the least populated statutory city in the country.

The city is an important cultural centre of the Polish minority in Trans-Olza, which makes up 12.1% of the population (as of 2021).

Třinec is notable for the Třinec Iron and Steel Works steel plant, the largest in the country, which still has a major impact on the city, its character, demographics, and air quality.

Třinec consists of 13 municipal parts (in brackets population according to the 2021 census):

The name Třinec is of topographic origin, derived from the Slavic word for reed (Polish: trzcina, Czech: třtina).

Třinec is located about 22 kilometres (14 mi) east of Frýdek-Místek and 30 km (19 mi) southeast of Ostrava. It lies in the historical region of Cieszyn Silesia, on the border with Poland and also near the border with Slovakia, which is about 20 km (12 mi) of the city centre.

Třinec lies on the Olza River. The Tyra Stream flows across the municipal territory and joins the Olza in the city centre. The elevation of the built-up area is approximately 300 m (980 ft) above sea level. The northern part of the municipal territory is located in the Moravian-Silesian Foothills. The southern, sparsely populated part is located in the Moravian-Silesian Beskids mountain range, which also belongs to the Beskydy Protected Landscape Area. The municipal border runs along the peaks of several mountains, including Ostrý (with an elevation of 1,044 metres (3,425 ft) the highest point in Třinec), Smrčina at 1,015 m (3,330 ft), Šindelná at 1,000 m (3,300 ft), and Javorový at 1,032 m (3,386 ft).

Třinec has a humid continental climate (Cfb in the Köppen climate classification).