Till WW1 it was a part of Croatia and under Hungarian rule. Therefore Hungarian stamps were valid there (example). The city had a mixed population, compromising mostly of Croats, Italians and Hungarians. The number of Italians was rising rapidly and in the year 1910 the presented half of the total city population.
In the Treaty of London (May 4th, 1915) Italy did not claim Fiume, which was intended to be the port of Croatia. Under the same Treaty, however, Italy was due to receive Dalmatia. At the Peace Conference after WW1, USA president Wilson refused to recognize the Treaty of London as binding on himself, and proposed to give Dalmatia to Yugoslavia, leaving Fiume an international port within the Yugoslav customs regime. Italy stuck to its demand for Dalmatia and now claimed Fiume also. It was over this question that the Italian Delegation left the Conference. Meanwhile, the authorities of the new Serb-Croat-Slovene State had taken up the standpoint that in consequence of the severance of Croatia from Hungary, Fiume become part of Croatia again. Croatian officials took charge of the administration and Serbian troops occupied the town on November 15th. They were persuaded to withdraw on an assurance that no foreign troops would enter Fiume for three days. In spite of the assurance, Italian troops promptly disembarked and the administration was taken over by Italians. Some French and British troops arrived afterwards, but the Italians were always in charge. While the Big Four in Paris were evolving a scheme for making Fiume into an independent, demilitarized buffer state, Italian poet d'Annunzio and his voluntaries made the coup on September 12th, 1919, declaring independent Free State Fiume. The state started issuing own stamps (example).
Despite the coup, the negotiation of the future of the city went on until, in January 1924, the arrangement was reached, under which Fiume went to Italy in absolute possession. Since then regular Italian stamps came into use (example).
After the capitulation of Kingdom of Italy the city came under authority of Mussolini's short - lived state called Socialist Republic of Italy.
After WW2 Fiume was liberated by Yugoslav partisans
and became a part of Croatia once again. Provisional stamps (overprinted stamps
of Socialist Republic of Italy) were used in the area during 1945 and 1946 (PIC).
Later regular Yugoslav and Croatian stamps (after 1991) were used in the city.