Tshisekedi officially triumphed with 73.47 percent of the vote.
The archbishop of Kinshasa denounced the elections as a "gigantic, organised mess".
The security challenge remains monumental for Tshisekedi.
The president of the Democratic Republic of Congo (DRC), Felix Tshisekedi, was sworn in for a second five-year term on Saturday after sweeping elections that the opposition branded a sham while calling for protests.
With tensions simmering in the vast nation's volatile eastern provinces, the 60-year-old president known as "Fatshi" picked Kinshasa's 80,000-capacity Martyrs Sports Stadium for the ceremony.
The stands were packed well before midday with singing and dancing in full flow, with some 20 invited heads of African states in attendance.
"I solemnly swear... to defend the constitution and the laws of the Republic... to maintain its independence and the integrity of its territory," the re-elected president declared before the judges of the Constitutional Court, before receiving greetings from the traditional chiefs of the 26 provinces of the country.
"I am aware of your expectations," Tshisekedi then declared in his inaugural speech, referring among other things to unemployment, youth, women and national cohesion.
Tshisekedi's first swearing-in, in January 2019 after controversially defeating Joseph Kabila, took place in the gardens of the Palace of Nations, a solemn venue that has traditionally staged important official events.
Tshisekedi is the son of the late historic opposition figure Etienne Tshisekedi.
He became president promising to improve living conditions in the DRC -- which boasts mineral riches but has a largely underprivileged population of 100 million -- and put an end to 25 years of bloodshed in the east.
He has not kept those promises but this time around he campaigned strongly on his first-term achievements such as free primary medication, asking for another mandate to "consolidate" the progress.
'Gigantic, organised mess '
More than 40 million people were registered to vote on December 20 for a president, as well as national and regional lawmakers and municipal councillors.
Polling was officially extended by a day to account for multiple logistical snarls and continued for days afterwards in remote areas.
In the end, Tshisekedi officially triumphed with 73.47 percent of the vote.
Moise Katumbi, a former governor of the central region of Katanga, came a distant second with 18.08 percent.
Martin Fayulu, who says he was also robbed at the last presidential vote, garnered five percent.
Denis Mukwege, who won the Nobel peace prize for his work with rape victims, scored just 0.22 percent.
These candidates called for the election results to be cancelled. A protest planned for December 27 was banned and snuffed out by the police.
The archbishop of Kinshasa denounced the elections as a "gigantic, organised mess".
Accusations of fraud and an "electoral hold-up" have not abated, and the fear of violence remains real in a country with a dark political past.
There was a lull in fighting in the east during the elections, but violence has since resumed between the army and M23 rebels, backed by neighbouring Rwanda.
Since mid-December, troops from the 10-nation Southern African Development Community (SADC) have been discreetly arriving in the DRC.
They are taking over from an East African peacekeeping force whose mandate was ended by the DRC government, which alleged it colluded with the rebels instead of fighting them.
A senior army officer said on Tuesday that Kinshasa was counting on them to help re-capture ground from the M23 militia, which has conquered swathes of territory in the east since taking up arms again in 2021.