The present work examines the standard of living among the Polish municipalities of the Euroregion Baltic—an institution engaged in cross-border cooperation which is striving to improve the standard of living in border areas. The time span of the study extended to the first full year after Poland’s EU accession (2004), and to the 15th anniversary of the accession and the 20th anniversary of the establishment of the Euroregion Baltic (2019 in both cases). All 38 Polish municipalities (NUTS 5) of the Euroregion were covered by the study. Using a synthetic index of standard of living based on Hellwig’s development pattern method, the municipalities were grouped into four classes according to their index value. Hierarchical methods were used to identify which municipalities had the most similar standards of living. The highest standard of living was recorded for the small, tourism-oriented town of Jastarnia and for strong urban centers (Gdańsk, Gdynia, Olsztyn, and Elbląg). Rural municipalities, especially those situated near the Polish-Russian border (Kaliningrad Oblast), had lower standards of living. The results show a progressing polarization in the standard of living, manifested by a widening gap between first-class municipalities and the other classes. This stratification was attributed to multiple factors, including the endogenous potential of the communities.