0919 GMT September 22, 2018
Pinera, who governed Chile in 2010-2014, accepted the presidential sash from outgoing socialist president Michelle Bachelet in a ceremony in the Pacific port city of Valparaiso.
Pinera was elected in December with a strong mandate, marking a new rightward shift in South America after conservative leaders rose to power in Peru, Brazil and Argentina.
His second term begins as prices for copper exports are rising, bolstering government revenues and driving growth in Chile's $250 billion economy.
But he will face a divided Congress and a prickly leftist coalition that has vowed to fight his plans to lower taxes and "refine" Bachelet's progressive policies.
Presidents from across the region attended Pinera's swearing-in ceremony, including Brazil's Michel Temer, Mexico's Enrique Pena Nieto, Bolivia's Evo Morales and Argentina's Mauricio Macri.
The 68-year old Pinera and his wife waved at supporters as they passed through streets packed with cheering Chileans waving the national flag.
Pinera is scheduled to deliver a speech later on Sunday.