The vote was the first in Ostia since police dissolved the district's government two years ago due to its infiltration by the mafia, the first time such a step had been taken in Rome. It was also closely watched as a barometer of public sentiment ahead of a national election that must be held next spring.
A first round of voting on November 5th left the Five Star Movement going head to head with the centre-right coalition Brothers of Italy, with the ruling Democratic Party failing to make the run-off.
The Five Star Movement won just under 60 percent in the second round on Sunday, making its candidate Giuliana Di Pillo the new president of Ostia's council with 15 of her party colleagues sitting alongside her.
Writing on her Facebook page after the victory, Di Pillo said: "It's a victory of honest citizens, who have retaken the local government and who want to rebuild from the rubble".
Ora il X Municipio può davvero ripartire già da oggi ci rimboccheremo le maniche per ripristinare la cultura della legalità e della trasparenza lavoreremo con grande impegno per ripagare la fiducia di tutti i cittadini https://t.co/9f4zsOZs3h pic.twitter.com/XYfUTLAax8— Giuliana Di Pillo (@giulianadipillo) November 20, 2017
Pictured: Giuliana Di Pillo celebrating with party colleagues. "Now the 10th district can really start again; from today we will roll up our sleeves to reclean the culture with legality and transparency, we will work to repay the trust of all the citizens."
"It's evident that there's been a Five Star Movement effect and a Raggi effect," she continued, referring to Rome's Five Star mayor Virginia Raggi, who triumphed in elections in the capital last June. "The voters understood that the Five Star Movement is the only political force that has never governed this region and we will work to pay back their trust."
There will also be nine opposition councillors, including one from the neo-fascist CasaPound party, which has been accused of ties to Ostia's organized crime families.
Two days after the first-round vote, a member of the notorious Spada clan was filmed headbutting a TV journalist as he sought to question the mobster about his alleged support for CasaPound.
Roberto Spada, the brother of boss Carmine Spada, was arrested for the attack, which left Rai2 journalist Daniele Piervincenzi with a broken nose.
The incident turned an international spotlight on Ostia, where journalists and residents protested in defence of freedom of speech.
Mayor Raggi condemned the attack and pledged to crack down on crime.
Turnout for Ostia's second-round vote was low, at less than 34 percent.
But the result is still good news for the populist Five Star, which also doubled its share of the vote in a regional election in Sicily earlier this month.
The party's founder Beppe Grillo said at the time: "The adventure has only just begun."