The ship spent the 1930s conducting training cruises in the Mediterranean Sea, participating in naval reviews held for foreign dignitaries, and serving as the flagship of the Cruiser Division.

She also helped transport Italian volunteer troops that had been sent to Spain to fight in the Spanish Civil War return to Italy in 1938.

Trieste was also employed to escort convoys to supply Italian forces in North Africa; during one of these operations in November 1941, she was torpedoed by a British submarine.

She carried a pair of IMAM Ro.43 seaplanes for aerial reconnaissance; the hangar was located in under the forecastle and a fixed catapult was mounted on the centerline at the bow.

[1] On 16 May 1929 she joined Trento in the newly created Cruiser Division for a cruise in the northern Mediterranean Sea that lasted until 4 June.

On 6 and 7 July 1933, Trieste, Trento, and the four Zara-class cruisers held a naval review for the Italian dictator Benito Mussolini in the Gulf of Naples.

On 7 June, she took part in a major naval review held during the visit of German Field Marshal Werner von Blomberg.

On 5 May, another naval review was held in the Gulf of Naples, this time for the state visit of German dictator Adolf Hitler.

There, they met four Italian merchant ships on 15 October, which embarked 10,000 members of the Corpo Truppe Volontarie (Corps of Volunteer Troops) that had been sent to support General Francisco Franco's Nationalist forces during the Spanish Civil War.

[10] This compelled Campioni to commit the battleship Vittorio Veneto, which in turn forced the British cruisers to break off the action, allowing both sides to disengage.

[11] On 9 February 1941, Trieste sortied with the rest of the 2nd Squadron to search for Force H after the latter had shelled Genoa; the Italians returned to port without success.

[12] At 06:55 on the 28th, an IMAM Ro.43 floatplane launched by Vittorio Veneto located a British cruiser squadron, and by 07:55, Trieste and the 3rd Division had come within visual range.

Seventeen minutes later, the Italian cruisers opened fire from a range of 24,000 yd (22,000 m), initiating the first phase of the Battle of Cape Matapan; in the span of the next forty minutes, Trieste fired a total of 132 armor-piercing shells, though trouble with her rangefinders and the extreme range of the action prevented her from scoring any significant hits.

By about 11:00, Vittorio Veneto had closed the distance enough to open fire, prompting Sansonetti to turn his three cruisers back to join the fight.

While the two sides were still maneuvering, a group of British torpedo bombers from Crete arrived and unsuccessfully attacked Trieste and the rest of her division shortly after 12:00.

A combination of heavy seas and the presence of British warships forced the convoy to shelter in Palermo, Messina, and Augusta in Sicily before being able to make the crossing to Tripoli.

On 22 August, Trieste sortied with other elements of the Italian fleet to try to locate Force H; they returned to port four days later empty handed.

[17][18] Trieste escorted another convoy to Libya on 21 November in company with the light cruiser Luigi di Savoia Duca degli Abruzzi.

The two damaged vessels were escorted back to Messina by the cruiser Giuseppe Garibaldi and the destroyer Bersagliere, arriving at around 08:00 the next morning.

[19] On 10 April 1943, while moored in La Maddalena, Sardinia, Trieste came under attack from B-24 Liberator heavy bombers from the United States Army Air Forces.

There, the ship was righted, and upon inspection, the shipyard workers discovered that fuel oil that had leaked into the engine rooms had preserved the machinery.

The Spanish Navy purchased the hull and towed it to Cartagena and then to Ferrol in 1952 to convert Trieste into a light aircraft carrier.