The decision was taken at the 48th session of the World Heritage Committee, currently under way in Busan, South Korea, and running until 29 July.
Speaking shortly after the vote, Gdynia's deputy mayor Oktawia Gorzeńska said the moment was dedicated to the city's residents, who "every day take care of this exceptional heritage and will protect it for all generations."
She added that she wished "to express sincere gratitude" for the decision.
City authorities had campaigned for the listing, arguing that Gdynia's centre is unique on a global scale.
According to the city council, only a handful of cities worldwide have a downtown built in a uniform modernist style – and in this part of Europe, Gdynia is the only one.
The listed area covers roughly 88 hectares and includes nearly 450 buildings.
In its description published on UNESCO's website, the centre is described as encompassing two principal street axes and interwar residential quarters, reflecting a large-scale application of modernist ideas.
Officials say Gdynia, a city on Poland's Baltic coast, was built quickly and ambitiously, guided by the architectural principle that "less is more."
The council describes the modernist buildings as marked by understated simplicity, functionalism and innovative construction methods, with large glazed surfaces lending lightness and brightening interiors.
Architects also drew inspiration from the sea, producing buildings resembling ships on land, complete with round windows, superstructures echoing captain's bridges, and walls curved like ship hulls.
Poland now has 18 sites on the World Heritage List, 16 cultural and two natural, including Warsaw's historic centre, the medieval town of Toruń and Malbork Castle.
The World Heritage List comprises nearly 1,250 sites globally.
Other additions at this year's session include the ancient Japanese capitals of Asuka and Fujiwara, seen as evidence of early state centralisation; the wilderness of Boma-Badingilo in South Sudan; Greece's Mount Olympus massif; and the D-Day landing beaches in Normandy.
(ał)
Source: IAR, PAP