This fact served to reaffirm the provisions of the Treaty of Versailles under which Poland took control over a 147-kilometre section of the coast. Gdańsk, together with its port infrastructure, remained outside the border and, as the Free City of Gdańsk, remained under thesupervision of the League of Nations. Dominated by the Germans, it was precluded from becoming a Polish maritime trading centre. Arguments for the construction of a new port also included numerous economic reasons e.g. possibility of bypassing the border barriers imposed by the neighbouring countries.

The choice fell on the region of the village of Gdynia, located at the mouth of the River Chylonka, where appropriate depth of the bay made it possible for ships to approach the coast, while the presence of a railway line ensured that goods could be transported to and from other locations in Poland. On 1 November, 1920, the Polish Economic Committee to the Council of Ministers granted an amount of 40 million marks to the Ministry of Military Affairs to construct a temporary military port and shelters for fishermen in Gdynia. Meanwhile, engineer Tadeusz Wenda started his work on the design of a commercial port capable of accommodating ocean-going ships. Works began in the spring of 1921. On 23 September 1922, the Legislative Sejm stated that the construction of the permanent port in Gdynia was one of the most significant tasks for the country. The works commenced in 1925; however, it was only in 1926 that they gathered momentum, as the position of the minister of industry and trade was taken over by Eugeniusz Kwiatkowski, an engineer.

During the same year, the construction of the city started to the south of the port. The plan for the city centre were drawn up on the basis of designs by Adam Kuncewicz and Roman Feliński, an urban planner from Lviv, who decided that the maritime “façade” of Poland - as Gdynia was called at the time - would be modelled on a sequence of squares in Nancy from the era of Stanisław Leszczyński. 10 Lutego street - together with an elongated square at its end, known as Kościuszki square - was to be the monumental axis of the plan. In 1936, after the completion of the Southern Pier which formed an extension of the said axis, the construction of a Grand District opening towards the sea also became a possibility, although the project was not finalised due to the outbreak of the World War II. The city centre, the construction of which was halted in 1939, features a unique urban layout, symbolically emphasising the link between Gdynia and the Baltic Sea.

The buildings comprising the centre originate mainly from the inter-war period; examples include the building of the Bank of Poland (1929) and the neo-Baroque church of the Blessed Virgin Mary - the Queen of Poland (1924-1927). However, it is Modernism that remains the dominant style in the area in question; the tenement houses from the 1930s in Świętojańska street, for example, are among the most interesting examples of this style anywhere in Poland. Some of the buildings in the city centre exemplify the then-popular tendency towards grand architectural forms (an example of this trend is the Cotton House, designed by W. Tomaszewski and erected in 1938); other buildings remain true to the Art Deco style (for example the building of the Meteorology Institute, designed by T. Doberski and W. Tomaszewski, 1927-1929). Maria Jolanta Sołtysik, a researcher examining the topic of Gdynia architecture, stated that the mid-1930s saw the rise in popularity of a luxurious variety of functionalism which combined modernity with elegance, attention to detail and opulent fittings, coupled with plentiful references to the design of modern ships. Outstanding examples of this trend include the office building of the Office Workers Insurance Institution (designed by R. Piotrowski, 1934) and the residential building of the Pension Fund of the Bank of National Economy (Bank Gospodarstwa Krajowego), designed by S. Ziołowski (1935-1937). The architecture of these buildings was international in character and took inspiration from designs of the most eminent European architects such as, for example, Erich Mendelsohn.

During that period, Gdynia became a place that attracted many distinguished architects. Apart from those already referred to above, one should mention K. Jakimowicz, who designed the former building of the Bank of National Economy (1928-1929) and Z. Karpiński, the designer of the Courthouse (designed in cooperation with T. Sieczkowski and R. Sołtyński, 1936).

Within several years, Gdynia was transformed from a fishing village into a thriving city. It obtained an official city status and saw a period of rapid development. With the passing of time, it became a significant Baltic port and a serious competitor for Gdańsk. During the brief interwar period, the Republic of Poland made a tremendous effort to create its own maritime policy and has subsequently managed to ensure its successful implementation. The very fact that Polish citizens - who had lived for centuries without caring much for the sea and the issues surrounding it - now became interested with sea travel, the navy and the national ensign was a great success in itself. Meanwhile, the city of Gdynia became a symbol of modernity - a status which it continues to enjoy even today.