The Siege of Castelnuovo, an engagement during the Ottoman-Habsburg struggle for control of the Mediterranean, takes place in July 1539 in the walled town of Castelnuovo, present-day Herceg Novi, Montenegro.

Castelnuovo had been conquered by elements of various Spanish tercios the year before during the failed campaign of the Holy League against the Ottoman Empire in Eastern Mediterranean waters.

The walled town is besieged by land and sea by a powerful Ottoman army under Hayreddin Barbarossa, who offers an honorable surrender to the defenders.

These terms are rejected by the Spanish commanding officer Francisco de Sarmiento and his captains even though they know that the Holy League's fleet, defeated at the Battle of Preveza, cannot relieve them.

Barbarossa's army suffers heavy losses during the siege due to the stubborn resistance of Sarmiento's men.

However, Castelnuovo eventually falls into Ottoman hands and almost all the Spanish defenders, including Sarmiento, are killed.

The loss of the town ends the Christian attempt to regain control of the Eastern Mediterranean.

The courage displayed by the Old Tercio of Naples, however, is praised and admired throughout Europe and is the subject of numerous poems and songs.