Alongside him was first lady Rosario Murillo, sworn in for her second term as his vice president.
"We are going to continue fighting to defend the people so they have health care, education and housing," the former Sandinista commander said in the capital's Revolution Plaza filled with the waving flags of his party.
Ortega, 76, and Murillo, 70, oversaw the jailing of opposition leaders, including seven potential challengers for the presidency, months before the November election. They have remained defiant under foreign pressure.
On Monday evening, Ortega called for the lifting of sanctions against Venezuela and Cuba — both of whose leaders attended the event — and said US President Joe Biden "has more than 700 political prisoners" in reference to those jailed in relation to the storming of the US capitol a year ago.
Ortega maintains that huge street protests against his government in April 2018 were an attempt to overthrow his government with foreign backing.
Earlier Monday, the US Treasury Department slapped sanctions on more Nicaraguan officials.
The Treasury Department announced it will freeze the US assets of the defence minister and five other officials in the army, telecom and mining sectors. As with dozens of Nicaraguan officials already under sanctions, US citizens will be prohibited from having dealings with them.
"Since April 2018, the Ortega-Murillo regime has cracked down on political opposition and public demonstrations, leading to more than 300 deaths, 2000 injuries, and the imprisonment of hundreds of political and civil society actors," according to a Treasury Department statement.
"More than 100,000 Nicaraguans have since fled the country."
The State Department is also imposing visa restrictions on 116 individuals linked to the Ortega regime, "including mayors, prosecutors, university administrators, as well as police, prison, and military officials".
The Ortega regime has been hit by rounds of condemnation and sanctions since the vote.
Nicaragua's government announced in November it will withdraw from the Organisation of American States, after the regional body accused Ortega's government of acts of repression and rigging the election.
The OAS General Assembly voted to condemn the elections, saying they "were not free, fair or transparent, and lack democratic legitimacy".
Twenty-five countries in the Americas voted in favour of the resolution, while seven — including Mexico — abstained. Only Nicaragua voted against it.
