The theater was built at the end of the first century at the instigation and financial support of the Triestine patrician Quintus Petronius Modestus, a co-commander of Emperor Titus in the Jewish War and procurator and flamen under Emperor Trajan. The stage was originally located on the seashore and served as a venue for theatrical performances.
The theater was probably destroyed by the Longobards in 568 who raided and besieged the city this year. The remains of the theater were then hidden for centuries under the medieval city walls and adjacent houses. Only in 1938, the ruins were rediscovered in the course of a major urban redevelopment. Mussolini then demolished a whole residential area of the old town. By exposing the Roman ruins, the continuity of the new fascist empire was demonstrated and the affiliation of Trieste to the Roman Empire and thus to Italy was demonstrated. Medieval parts of the city were destroyed in the course of the excavations.
Today, the Teatro Romano is an indication of the wealth of Trieste at the time of the Roman Empire and serves as the venue of a festival in July every year, the Teatro Romano Festival.