Gdynia is a city in northern Poland and a seaport on the Baltic Sea coast. With an estimated population of 257,000, it is the 12th-largest city in Poland and the second-largest in the Pomeranian Voivodeship after Gdańsk. Gdynia is part of a conurbation with the spa town of Sopot, the city of Gdańsk, and suburban communities, which together form a metropolitan area called the Tricity (Trójmiasto) with around one million inhabitants.

Historically and culturally part of Kashubia and Eastern Pomerania, Gdynia for centuries remained a small fishing village. By the 20th-century it attracted visitors as a seaside resort town. In 1926, Gdynia was granted city rights after which it enjoyed demographic and urban development, with a modernist cityscape. It became a major seaport city of Poland. In 1970, protests in and around Gdynia contributed to the rise of the Solidarity movement in nearby Gdańsk.

The port of Gdynia is a regular stopover on the cruising itinerary of luxury passenger ships and ferries travelling to Scandinavia. Gdynia's downtown, designated a historical monument of Poland in 2015, is an example of building an integrated European community and includes Functionalist architectural forms. It is also a candidate for the UNESCO World Heritage List. Its axis is based around 10 Lutego Street and connects the main train station with the Southern Pier. The city is also known for holding the annual Gdynia Film Festival. In 2013, Gdynia was ranked by readers of The News as Poland's best city to live in, and topped the national rankings in the category of "general quality of life". In 2021, the city entered the UNESCO Creative Cities Network and was named UNESCO City of Film.

The area of the later city of Gdynia shared its history with Pomerelia (Eastern Pomerania). In prehistoric times, it was the center of Oksywie culture; it was later populated by Lechites with minor Baltic Prussian influences. In the late 10th century, the region was united with the emerging state of Poland by its first historic ruler Mieszko I. During the reign of Bolesław II, the region seceded from Poland and became independent, to be reunited with Poland in 1116/1121 by Bolesław III. In 1209, the present-day district of Oksywie was first mentioned (Oxhöft). Following the fragmentation of Poland, the region became part of the Duchy of Pomerania (Eastern), which became separate from Poland in 1227, to be reunited in 1282. The first known mention of the name "Gdynia", as a Pomeranian (Kashubian) fishing village dates back to 1253. The first church on this part of the Baltic Sea coast was built there. In 1309–1310, the Teutonic Order invaded and annexed the region from Poland. In 1380, the owner of the village which became Gdynia, Peter from Rusocin, gave the village to the Cistercian Order. In 1382, Gdynia became property of the Cistercian abbey in Oliwa. In 1454, King Casimir IV Jagiellon signed the act of reincorporation of the region to the Kingdom of Poland, and the Thirteen Years' War, the longest of all Polish-Teutonic wars, started. It ended in 1466, when the Teutonic Knights recognized the region as part of Poland. Administratively, Gdynia was located in the Pomeranian Voivodeship in the province of Royal Prussia in the Greater Poland Province of the Kingdom of Poland and later of the Polish–Lithuanian Commonwealth. The present-day neighbourhood of Kolibki was the location of the Kolibki estate, purchased by King John III Sobieski in 1685.

In 1772, Gdynia was annexed by the Kingdom of Prussia in the First Partition of Poland. Gdynia, under the Germanized name Gdingen, was included within the newly formed province of West Prussia and was expropriated from the Cistercian Order. In 1789, there were only 21 houses in Gdynia. Around that time Gdynia was so small that it was not marked on many maps of the period: it was about halfway from Oksywie and Mały Kack, now districts of Gdynia. In 1871, the village became part of the German Empire. In the early 20th century Gdynia was not a poor fishing village as it is sometimes described; it had become a popular tourist spot with several guest houses, restaurants, cafés, several brick houses and a small harbour with a pier for small trading ships. The first Kashubian mayor was Jan Radtke. It is estimated that around 1910 the population of Gdynia was 895 people.

Following World War I, in 1918, Poland regained independence, and following the Treaty of Versailles, in 1920, Gdynia was re-integrated with the reborn Polish state. Simultaneously, the nearby city of Gdańsk (Danzig) and surrounding area was declared a free city and put under the League of Nations, though Poland was given economic liberties and requisitioned for matters of foreign representation.

The decision to build a major seaport at Gdynia village was made by the Polish government in winter 1920, in the midst of the Polish–Soviet War (1919–1921). The authorities and seaport workers of the Free City of Danzig felt Poland's economic rights in the city were being misappropriated to help fight the war. German dockworkers went on strike, refusing to unload shipments of military supplies sent from the West to aid the Polish army, and Poland realized the need for a port city it was in complete control of, economically and politically.

Construction of Gdynia seaport started in 1921 but, because of financial difficulties, it was conducted slowly and with interruptions. It was accelerated after the Sejm (Polish parliament) passed the Gdynia Seaport Construction Act on 23 September 1922. By 1923 a 550-metre pier, 175 metres (574 feet) of a wooden tide breaker, and a small harbour had been constructed. Ceremonial inauguration of Gdynia as a temporary military port and fishers' shelter took place on 23 April 1923. The first major seagoing ship, the French Line steamer Kentucky, arrived on 13 August 1923 after being diverted because of a strike at Gdansk.