The market is the meeting place of supply and demand, and their characteristics determine the market structure. Market structures range from perfect competition, characterized by a large number of producers and consumers, all the way to the monopoly market structure, characterized by a single producer. Industrial concentration indicators based on companies' market shares in the industry are used to determine the market structure. Indicators used in this paper are concentration ratio of the 2 largest companies in industry (CR2), concentration ratio of the 4 largest companies in industry (CR4) and Herfinndahl-Hirschman index (HHI). Focus of this research is the market of nautical tourism ports of the Republic of Croatia in 2019. Industrial concentration indicators imply that nautical ports in Croatia have loose oligopolistic market structure. The concentration ratio of the 4 largest companies (CR4) is 54.74%, and the Herfinndahl-Hirschman index (HHI) is 1413.15., both indicating moderately concentrated market. Although the calculated industrial concentration indicators imply a loose oligopoly, the nautical tourism ports market does not have all its characteristics but is inclined to monopolistic competition in many ways. The government has a significant role in the market as a provider of concessions on maritime assets, but also as the owner of the largest company, Adriatic International Club Croatia Plc. (ACI).