Because of the increased exposure to pirate raids, the center had low priority during the rule of Aldobrandeschi and of the Republic of Siena, but with its entry into the State of Presidi in the mid-16th century, the town became a center of great importance in Argentario. It was during this the construction of the Spanish Fort began, a powerful defensive structure that now hosts a permanent exhibition "Submerged Memories". Like all other centers in the area, Porto Santo Stefano joined the Grand Duchy of Tuscany in the first half of the 19th century.
Besides the fortress, the territory still plays host to two Spanish lookout towers; Lividonia and dell'Argentiera, the latter situated on the slopes of Mount Argentario which dominates the whole Santo Stefano area.
Notable events in the town include the Palio Marinaro, an annual 4,000-metre (2.5-mile) rowing regatta – a boat is called a "gozzo" (plural "gozzi") – which is held every August between the four districts. The prize was started in 1937, but discontinued from 1940 to 1944 because of World War II. The list of victories are: 23 victories for the Pilarella district, 19 for the Cross district, 16 for the Valley and 13 for the Fortress district.
Transportation and infrastructure
Porto Santo Stefano was once connected by rail to Orbetello via a small rail line, closed in 1944. The town has two port harbors: Porto Vecchio and Porto del Valle.
Porto Santo Stefano has regular ferry service to the Isola del Giglio (Giglio Island). After the capsizing of the cruise ship Costa Concordia on 13 January 2012, many of the ship's passengers and crew were evacuated to the mainland on these ferries.