Seinäjoki (Finnish: [ˈsei̯næˌjoki] ^ⓘ; lit. "Wall River"; Latin: Wegelia, formerly Swedish: Östermyra) is a city in Finland and the regional capital of South Ostrobothnia. Seinäjoki is located in the western interior of the country and along the River Seinäjoki. The population of Seinäjoki is approximately 67,000, while the sub-region has a population of approximately 132,000. It is the 16th most populous municipality in Finland, and the 13th most populous urban area in the country.

Seinäjoki is located 80 kilometres (50 mi) east of Vaasa, 178 kilometres (111 mi) north of Tampere, 193 kilometres (120 mi) west of Jyväskylä and 324 kilometres (201 mi) southwest of Oulu.

Seinäjoki originated around the Östermyra bruk iron and gunpowder factories founded in 1798. Seinäjoki became a municipality in 1868, a market town in 1931, and a town in 1960. In 2005, the municipality of Peräseinäjoki was merged into Seinäjoki, and at the start of 2009, the neighbouring municipalities of Nurmo and Ylistaro were amalgamated with Seinäjoki. The city itself is growing steadily from year to year, but the population of the surrounding municipalities is correspondingly decreasing.

The city hall, city library, Lakeuden Risti Church and other public buildings were designed by Alvar Aalto. Seinäjoki was historically called Östermyra in Swedish. Today, this name, which was never official, is very seldom used even among Swedish speakers. Seinäjoki Airport is located in the neighbouring municipality of Ilmajoki, 11 kilometres (10 mi) south from the Seinäjoki city centre. Seinäjoki railway station in city centre was opened in 1883 and until 1897, it carried the name Ostermyra station.

The settlement spread in the area of the present Seinäjoki during the first half of the 16th century. During the 1550s, there is said to have been three houses in Seinäjoki: the houses of Marttila, Jouppi and Uppa. The house of Jouppila, which separated from the house of Jouppi, was established during the same century. All of the houses were located on the shore of the river.

Seinäjoki belonged to the church parish of Ilmajoki like Kurikka, Kauhajoki, Jalasjärvi and Alavus. However, in the 18th century the roads from Seinäjoki to the Church of Ilmajoki were generally in poor condition. Therefore, the inhabitants of Seinäjoki and the neighbouring Nurmo built a new chapel together in 1725, which in 1765 led to the formation of the chapel town of Nurmo. Seinäjoki, which was called Alaseinäjoki since the Greater Wrath, became a part of the chapel town. The chapel parish of Peräseinäjoki was founded in 1798, and the village of Alaseinäjoki began to be called Seinäjoki again. The very same year, the Östermyra steel mill was founded on the shore of the Seinäjoki river.

In the 1850s, actions were taken to separate Seinäjoki from the church parish of Nurmo. Ilmajoki wanted to connect Seinäjoki back to its own parish. In spite of strong objections from the inhabitants of Nurmo, the Senate of Finland accepted the petition from the inhabitants of Seinäjoki in 1863, to form a chapel congregation of their own. Seinäjoki got an independent local government in 1868. In 1900, Seinäjoki became an independent municipality.

Seinäjoki has grown around a few important railroad crossings. The Tampere–Vaasa railway, which passes through Seinäjoki, was inaugurated in 1883. The track, along with the Kokkola track that was opened for rail service in 1885, and the Kristinestad track which had been completed in 1913, raised Seinäjoki as an important railway crossing section in Finland. In the early 1970s, a direct railway between Tampere and Seinäjoki was opened, and the services of Seinäjoki improved further.